Just to echo Corrisive's advice as its very apt; watch out for clients who try to barter you down on price constantly (unless you think they have good reason to), or give the oldest line in the book 'if you do it for this price now, there will be plenty more of where that came from' wink wink, offer to pay you in cash, or just generally come across as a bit of a wideboy/dodgy.

I had an experience with a client once who fit all the above and he was such a nightmare to work with in the end I was glad to see shot of him. As you can imagine I only got about 1/3 of the money owed. On the positive side it happened when I was starting out and I learnt a lot from it.