Once you feel like you've got a sense of what "normal" prices are for cars in your price range, get a consumer reports subscription†. Use CR to evaluate the make, model, and year of any car that you're considering for safety and reliability. Reliability is a pretty important indicator for cars in this price range, because you'll tend to be looking at things with higher milage.
Then, watch craigslist for vehicles that seem like a good deal. With the information you have, you should also be in a good position to call local used car dealerships and tell them what you're looking for. You should have enough information to tell when they're offering you a reasonable deal.
If you have the time and can afford to try and be a little picky. Obviously, if you need a car you need a car, but if you can afford to take your time it'll usually be worth it. Prefer cars that seem like they've been well-maintained over their life and prefer cars that come from people who have kept documentation of maintenance performed on the car.
Finally, no matter where you're considering buying from—dealer or individual—take the car to a mechanic for a pre-sale inspection. This is a common service most mechanics offer. Make sure you're the one selecting the mechanic and the one paying the mechanic. That ensures that you should get a fairly objective assessment of the car itself. This step isn't as important if you're familiar with cars, but if you're not a car guy (as I'm not), it's totally worth the price to find out if there are any immediate or upcoming service issues that will be necessary for the car.
†I _think_ you can get a monthly subscription and cancel after one-month, but you should double-check that. If you can get it for a single month, the $7.95 is totally worth it. If you have to get it for a full-year at a time, evaluate the cost/benefit based on your budget.
