More art-for-pay advice to keep you from following my trail of stupid mistakes.
Contracts

I generally don't use a contract if the art I'm going to do is not for commercial purposes or the cost is under $100.
What is art for commercial purposes? If the commissioner or client is going to use this image for monetary gain or advertising, that makes it commercial. I know the actual laws on the subject are more complicated, but that's a quick rule of thumb I use. Be wary of a client who doesn't want your signature on it. Even if they say they're not using it for commercial purposes, the unwillingness to let you sign your work may be an indication that they're not being entirely honest. A contract may be useful in this instance just in case.

It's not rude to ask for a contract. You are protecting yourself *and* your client. Any client with an interest in being fair will not object. You're not being pretentious. You may feel awkward asking for one at first, but it does add a level of professionalism to your standard practices that can make both you and your client feel more comfortable in the end.

Make sure it's a contact with terms that are comfortable for everyone. Both parties may have to compromise in the end, but "comfortable" doesn't mean "everyone gets everything they want", it means "Okay, we can both agree to these terms".

If the client provides you with a contract, you *must* read it first. There could be something in there that cuts your rights, restricts your pay, or imposes unreasonable deadlines.

It's okay to ask for additions and revisions. Don't ever feel guilty about it. You have just as many rights as your client and you should never feel bad about protecting yourself, your time, and your interests. I'm the kind of person who has trouble speaking up, but I'd rather gather the courage and take that chance than go through an entire job feeling cheated and voiceless.

If you are unsure about the contents of a contract or have questions, ask for clarification. The client shouldn't be afraid to explain. If they are, then you may be better off walking away. You can also ask other professionals you know who have dealt with contracts before. Outside of a lawyer, no one can give you definitive legal advice, but someone who has worked with art contracts before can give you their impression of things based on their past experience. I don't know many professionals who wouldn't be happy to take the time to make sure that others in their profession are being treated fairly. For example, I would be willing to look over contracts. I'm not a lawyer, but I've dealt with contracts and written them and may be able to provide some insight.

A complete contract should include the following:
1. Defined pay rate and pay date.
(What you are to be paid and when you are to be paid. Even *how* you are to be paid. Personal checks aren't always reliable.)
2. Defined dates.
(When you are to start and when the art is due.)
3. Defined work hours.
(How many hours a day you will work, what hours they are, how many days off you need. I recommend five days a week, eight hours a day with appropriate breaks, and an addition of any day or holiday you may also need for yourself. Make sure the client understands that you may be contacted during these hours only. Make sure it works with your schedule and theirs.)
4. Expectations about revisions, corrections, and editing.
(This makes it so you aren't told to change something at the very end that *should* have been changed at the beginning. Revisions can put a strain on your time and your sanity. I like to say that once a sketch is approved, no changes can be made that would involve further sketches, same with inks, colors, and finished product. Once it's been approved, it's approved. You may be more comfortable with revisions later in the process, and that's fine. The point is that everyone knows what is acceptable and expected of them.)
5. Definition of "client" and "contractor".
(Who you are, or the names of all the people you are working with. Who your client is, or the names of all parties who have the right to approve or make revisions.)
6. Definition of who may do what with all the art.
(Chances are you won't get to post it anywhere and all of the art, from sketch to finished piece, are the sole property of the client to do with as they please. There's nothing wrong with that so long as that's the agreement. If you want to use the piece for your own purposes -as a portfolio piece or to post on your website/DA, it's their decision whether or not that is allowed. At the end of the day, this is their legal and intellectual property. Their rights need to be respected as well.)
7. The signatures of everyone involved.
(And I do mean *everyone*. I've had a client say "well, I have a silent partner who wants these changes. I know it's outside of the contract, but he never signed it". That's bullshit. You have a right to know who is involved and you all have to agree to all the terms. Don't forget to include the date!)

The most important thing of all is that the contract is signed by all parties before you begin.
Do *not* turn over a single sketch or concept before it's signed! If you are asked for any work before you have a singed and finished contract for your records, be firm and say "I need that contract first." You're not being rude or picky. You're being professional.
For the sake of covering my own ass, I'm not accusing anyone I've worked with of shady practices. Chances are that the fault has been with my lack of experience.
Get it in writing!
-Nuisance
If it's over $100, I will use the regular contract.
Being professional isn't easy, but it will save your ass and a lot of necessary trouble!