You can read through this forum and find a LOT of thoughts and suggestions. I've learned a lot from my 1st surrogacy to my last and have tweaked things here and there. Some for me have been changing the payment structure (1st time it was trimester payments, after that it was equal monthly payments), changing when a multiples fee was started to be paid out (late 2nd tri or 3rd tri is not acceptable to me), making sure that 'child care' was NOT limited to only a licensed provider/daycare facility/etc. (both my mom and DS#1 have had to step up, and I wanted to make sure that any family/friend that provided any care could be reimbursed) and that there were no caps on my lost wages, in the event of bedrest. I was specific as to how far along I might be that I would agree to termination and under what conditions I would have one. After having carried twins, I made sure my contracts reflected that I would only do a SET also.
Travel restrictions is another. Some are severely limited earlier on in the pg and have to travel for their job, so they get screwed. I live in a boarder town, as do my parents in the state next to mine. The closest hospital in my state from my parents is 30mins (less w/lights and sirens), so my contract stated that I couldn't travel beyond where they live after 'x' number of weeks, but because it was so close, the limit was like 36wks I believe. Some will state you can't go more than 70mi from the hospital you'll be delivering at, or some variation of that. This IS important because of the legal and financial implications for the IP's and their legal parentage of their own child.
Food is another. Some IP's request very strict dietary restrictions of their surrogate, others are more liberal and trust the surrogate. Make sure that you understand their expectations when you are matching and read over the contract very closely. There will always be restrictions on amt of caffeine, often fish, sushi, etc, but it is not unheard of to see some pretty crazy things listed to either avoid or that they want the surrogate to eat. (nationality can also play a big part in dietary demands)