I know my form isn't terrible, but it certainly isn't perfect either. How do you work on improving it? Are there certain exercises that will strengthen the right muscles and force better form? Do you concentrate on footfall while running?
The number one thing you can do is train with a coach who knows about good running form. Many cities have various track clubs that are friendly to people who are less than lightning fast. However, that costs money, takes time, and may not be ideal.
One activity that I used to learn a lot about my running style was setting up a video camera to record me running on a treadmill. I set up the treadmill to have me run in a couple of different ways, about the pace I use for my long slow runs and my "racing pace", both with and without shoes. I then stepped through each of the videos frame by frame to get an idea for how my foot was hitting and rolling as I ran. Unfortunately, because the video was from the side, it made it hard judge whether or not my feet were actually hitting straight on and how far left or right they were. After watching each video in slow motion, I stepped through the shoe and barefoot videos side by side to understand how my kick was working and, more importantly, what sort of pressure I was putting on my feet while landing. It wasn't pretty and made it obvious why my knee hurt so much, with shoes I was hitting on my heel with my leg fully extended. There was nothing to absorb shock except my heel.
Using that information I've been paying extensive attention to my running form while on training runs -- regardless of whether or not I'm barefoot. Mainly this consists of making sure that I land further up on the ball of my foot with a slight bend in my leg and that my heel hits the ground after the ball of my foot. An easy way to tell if you've done this right is by listening to to the noise that your footfall makes. Landing on your forefoot is usually much quieter.
This will, however, end up hurting your calves. It took me about four weeks before I was comfortable running with a forefoot strike without focusing on my food landing. My calves still haven't built up all the way, but that's coming.
For what it's worth, I got the idea of recording my running from a running analysis video by NJ Sports Medicine on YouTube.
The advice I was given by a professional physiotherapist was to concentrate on stride length, and landing on the back of the foot/heel (I pronate though, so this was probably foot shape specific). It may have been very generic advice and not something an Olympic champion would care about, but it helped my pace/lap time quite a lot.
Squats and glute exercises are worth considering too, especially if you are increasing your mileage fairly quickly, both help if you suffer from shin-splints.
You're probably past this point but the advice I give beginners about running form is to stop looking down while they run. My girlfriend improved her running form and speed remarkably in about 2 minutes just by following this advice.
When you look down while running, you lean forward too much, your feet land too soon and your stride becomes more of a shuffle.
Look well ahead like you do when you're driving a car. It keeps you upright and your feet will get a chance to move ahead of your body before they land.
If you're worried about tripping don't be. Because you're not shuffling anymore your feet come down to meet the ground at a higher angle and so don't hit the obstacles you used to hit when you were shuffling.
Assuming you are reasonably fit, the best way to improve your running technique is to run faster. Don't sprint, but run for example at the maximum pace you can keep for 10 mins (I run 12, which coincides with the Cooper test for VO2Max, but that's not so important). Running faster will automatically correct the small mistakes you have with landing, stride length, body position, even breathing - the body will automatically eliminate the little inefficiencies if you push it to the maximum. This is why you should be at least a decent runner, as a sudden 100% effort is not nice and could be dangerous. The pace is something you'll have to judge for yourself (stay safe!) but you can try a 10 min run at a slightly higher speed and then work up from that as you see fit.
The other answers still apply, but everything is easier if you don't have to think consciously.
The thing that has helped me the most with my running form is running with as minimal of footwear as I can stand. I usually run in either water shoes (like you wear on the beach) or huarache sandals.
I find that when I run in shoes, I fatigue quicker, don't seem to run as freely, and my steps have less spring in them.
Running minimalist makes me realize how many muscles in my legs, calves, and feet are weak and underutilized. Perhaps bad running form is the body's way of compensating for these weaknesses.
It seems that improved running form comes more naturally when I run closest to barefoot.