-Focus on the eyes. If your subject's eyes are sharp, the whole picture is likely to appear "in focus" even if it's not. In fact, for close ups, the nose, lips, and other features are very often well out of the focus plane if you're using a wide aperture. And I don't mean focus between the eyes on the bridge of the nose. Choose one eye (the one closest to you) and put your focus point on the iris. At large apertures even an inch can be the difference between sharp eyes and soft eyes. Put your finger on the bridge of your nose and then move it to your eye and you'll see what I mean...they're not on the same plane. But beware, if your subject is at all angled, the eye furthest from you will be out of focus at large apertures.
-For full body shots, try a longer focal length (85-200mm is good) so you can use a wider aperture and keep your subject in focus.
-Try different methods for choosing your focus point to see which you prefer.
There are multiple options when it comes to focusing and which one you choose really depends on your personal preference and what type of pictures you're taking. Personally, when it comes to fast-moving kids, I tend to stick with the "center point/recompose" method (if you didn't even know that you COULD choose your focus points, pretty please read your manual).
-Don't let your camera choose your focus points! Why have your camera guess what you want to focus on when you KNOW what you want to focus on. Letting your camera choose is a sure-fire way to get a lot of soft pictures.
-Know what works with your focus method and what you might need to do differently. For instance, with most cameras you'll have to take the extra step of locking in your exposure before recomposing your shot if you are spot metering. Don't do it and your exposures will all be off since as you recompose the shot, your camera will remeter.
-Try toggling your focus points. If like me you usually use your center point, but your images aren't as sharp as you'd like, try toggling your focus point and positioning your subject so that you don't have to recomose the shot. Some people have better luck getting sharp pictures with this method, but it takes some practice to get quick at it. It's especially wise to toggle when taking portrait oriented pictures since the center point is usally a far distance from your subject's eyes.
-Experiment with your camera's AF modes and back button focusing. Most dslr's have multiple modes to choose from, some that focus on just what you tell it to, and others that will "follow" your subject if they're moving to keep them in focus. I haven't had much luck with the latter (AI-Servo), so I stick with One Shot AF mode. Some people also love back button focusing which lets you take the AF function away from the shutter button. Google it, you'll find ample instructions.
-Look large. If you're only used to looking at your photographs on a small monitor, try looking at a 100% view to see if things are really as sharp as they appear. Sometimes a picture will look great at a small size like what's on your monitor, but blown up all of the imperfections start to jump out at you. Now, don't get too obsessed with this, the eyes on a full body shot will never appear as sharp as those on a close up, but that's OK, it's just a matter of how many pixels you're working with.
I think part of learning how to focus is also learning what you want out of focus. In fact, most people just want to know how to get a blurry background. I'm not going to get into explaining DOF, there are already many articles online that can do that, but I will give a few more pointers.
-Use a larger aperture if you're working with just one subject since it decreases the depth of field. Just remember to focus wisely (i.e. on the eyes) as there's little room for error.
-Invest in lenses. Lenses make a lot more difference in the quality of your pictures than your camera body does. The most expensive lenses are usually the ones that allow for the widest apertures. If your lens only goes down to f4 you aren't going to get the same results as a lens that goes down to 1.8 or lower. If you're looking for a cheap lens to experiement with for portraits, try a 50mm 1.8, it can be had for under $100! Prime lenses (ones with one focal length) are also known for being sharper and faster than zoom lenses.
-If you're photographing a family or muliple subjects, use a depth of field calculator (google it, there are many) to determine your best aperture for keeping everyone in focus. I'll often take one shot at the largest aperture I think I can get away with, and one at a "safe", smaller aperture.
-Use a longer focal length so that you can stand back and zoom in on your subject or the group. This will help with keeping a blurry background and allows for a larger aperture.
-Keep your subjects on the same plane, or close to it, to allow for a larger aperture. If you have parents sitting behing kids you'll need a much smaller aperture to keep them all in focus, which means your background will be a little sharper too.
-For really nice bokeh, look for the trees! Trees or tall bushes with light coming through them often make for the best bokeh. If you just have sky, or a plain background, you won't notice much.
-Don't get discouraged! If you start experimenting with different methods of focusing, and with using larger apertures, you'll probably have a lot of out of focus shots at first. It takes some practice. Keep at it, find what works best for you, and you WILL start to see improvement. You'd be surprised at how many shots most pros take and how many end up out of focus, especially with moving kids!
Now that you've gotten a few pointers, here are some examples to take a look at:
In this first example, I love the picture, but if you were able to see it bigger, you'd see that the focus is off. Now, you might be saying, eh, looks OK to me, but scroll down a bit and see the close up of her eyes. On screen, it might not appear too bad, but blown up on a wall canvas it would be a lot more obvious. 
Looking back at my settings (ISO 400, f/2.8, 1/80s), this was likely just a case of shooting with too slow of a shutter speed along with a little hand shake. A blow up of the whole picture shows that her entire body is out of focus, rather than just her eyes. If her eyes had been out of focus, but her nose had been tack sharp, I'd blame it on a missed focus point. To correct it, I could have opened up the aperture to 2.0 or even 1.8 to increase the shutter speed, or I could have bumped the ISO up to 800 which may have led to the picture being a little noisy, but again, a faster shutter speed. 
Here's an example that's a little tougher to see at this size, but you might be able to still tell the difference between this picture and the one below it. While I love the composition, colors, and light in this photo, it's probably not one I would show to a client in a proofing gallery. Why? The eyes. At this size or smaller, it doesn't look too bad, but if I wanted to blow this up to anything larger than a 5 x 7 the eyes would be obviously soft, or out of focus. This was shot wide open at 1.8 which leaves little room for error when it comes to your focus point. One inch to the left or right can mean that the bridge of the nose is in perfect focus while the eyes are not. 
Now take a look at this photo and you'll see that his eyes are tack sharp. They may actually appear to be TOO sharp when you view them on screen because of the file size. This was taken with the 85mm at f/2.0, so still a very large aperture with little room for error. But this time, my hand must have been a little steadier, the focus point was right on, and I had a fast shutter speed.
Here you can really see the advantage of shooting with a large aperture when doing full body portraits. The background in this photo, taken at f/1.8 just melts behind him into a blurry tunnel of trees rather than being distracting. You can see how the subject is clearly the focus if the picture, almost popping out in 3D. Not everyone likes the look, much of it is personal preference and artistic choice, but personally I love it. It took a lot of practice to get it right, especially at 50mm as in this shot, but when you get it, it's worth it. And by all means, don't get discouraged if half of your shots, or more, aren't sharp when you try shooting at large apertures. Camera shake, subject movement, low shutter speed, etc. can all make it really tough to get every, or even the majority of shots sharp.
This shot, while not the best in composition, is a good example of bokeh. You can see how everything behind his is creamy, and where the light meets the trees there are nice circles of color. This is where more expensive lenses can really make a difference. This was taken with the 50mm 1.8 which, while a great cheap lens for most purposes, isn't known for having the best bokeh. When viewed closely, the circles are actually pentagons due to it's 5 blades. Nevertheless, I like it!
This is an example of how choosing your aperture while shooting groups or families can really make a difference. This was taken at f/4.5 which is clearly not a small enough aperture to keep the whole family in focus, but that's what I was aiming for. Sometimes having a few members slightly out of focus can add interest to the picture, especially when it's a more candid shot like this. In this case there also simply wasn't enough light to allow for a small enough aperture at this angle to have everyone sharp. The shutter speed would have gone down to around 1/30 which is just too slow.Next time...composition!
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