Jef,
There will always be the risk of a contractor not preforming, even if they are good they can get so busy that there schedules become challenged.
When you do your job walk have all the contractors there, this will save you time and let them know you are serious about your numbers. Have a job summary written for each sub with a scope of work, this way there is no questions on what they are bidding.
Each sub should provide you with a referral list and I suggest you call them all, get a good feel for the work they have preformed, was the schedule met, how did they communicate, was the budget kept.
You can also call the contractors board to make sure there are no complaints, see if they are up on insurance and bonds.
Provide the subs with a schedule and make it part of your contract, by signing your contract they are then bound to this schedule.
You will typically have some money at risk with your subs, a percentage will be given to start and depending on the length of the job possibly a draw part way through, never pay in full until you have approved there work and are satisfied.