Excellent summation, Cyangmou!
I just wanted to stress the importance of trying to negotiate a $/hr rate, versus a fixed fee or $/sprite. This has a lot to do with communication and the potential for revisions. If you have an excellent client who has a well defined asset list it might not be such a big deal, but I've found most of those who are offering a flat rate per asset are generally inexperienced and not entirely certain what they want. Each time you have to go back and edit or redo a sprite or tile that is money out of your pocket. I learned this early on, when a client insisted on a flat quote for a project. I estimated it would take me a month to deliver, and priced it accordingly. Poor description and constant direction changes ended up drawing that contract out to three months, and as I had signed a contract to complete it I had a legal obligation to complete the job to the client's satisfaction. At the end, I was essentially working for less than minimum wage, and that was the end of that type of contract for me.
As a client, I would insist on at least weekly email updates. Artists are flaky, and that one email can help keep them on track and motivated.
I'm still pretty crap at insisting on contracts, though. I'm far too trusting, even after having my fingers burned on a few occasions.