Getting a used car is a great idea nowadays when you consider all the great offers that you can find from most dealers – but of course, this not only depends on where you live, but also how savvy you are with shopping and bargaining. Thankfully, these days it's a bit easier to compensate for a lack of negotiating skills in most cases, so you shouldn't have to worry so much about that – and instead concentrate on finding the best possible car for your current needs.
Try to figure out what your financial capabilities are from the very beginning, and set a hard limit for yourself – this is important, as most dealerships would do their best to make you pay more than you've initially mentioned to them, and it's important that you stand your ground and know where your limits are.
On that note, don't mention what your limit is to the vendors that you're working with – you won't benefit anything from it, and keeping the exact sum a mystery can give you more leverage when you're doing the negotiations, as you'll be able to play with the price in a more flexible manner. Just remember that the salesperson will have their limits as well, and you must be careful to avoid going over them in an annoying way – as this may result in an unpleasant end to your bargaining, without you getting the car that you were hoping for, or at least not for that ideal price that you saw earlier.
In addition, it's a very good idea to know a good mechanic who can do a check-up on the car for you. This should be done immediately before closing the deal if possible, as it will give you some extra leverage for the negotiation of the price – if there are any flaws which the mechanic can take care of later on, you can usually have the price of the car decreased significantly (depending on the exact kinds of flaws, of course), so ensure that the car is looked through thoroughly.