The more important thing than signing with those who are obviously trustworthy, is that in that all-powerful contract negotiating time, read the contract thoroughly. Go over it with a fine-tooth comb. Look for the slightest ambiguity. Make sure that it is detailed out, point by point, exactly what you will have done when.
Take it to your lawyer ahead of time, and work with them to find any loopholes you missed. Even if there's no such thing as an ironclad contract, you can make it as close as possible. Be cautious of industry lingo. You may understand that "demo" means something on the floor, but make sure you know exactly what kind of demo, and get it in the contract.
Push for a stipulation that all discussions about the work be handled in written format (email or snail mail, doesn't matter) so you have a strong paper trail.
If you do all this, then it will be harder for them to find loopholes to push you on, and you'll have a paper trail of any changes they tried to pull last minute.