Short answer is you can't. For small transactions, the best form of contract is a handshake and trust. Don't pay in advance, or do work in advance of payment, for more than you are prepared to loose. Make payments based on outcomes, at a granularity that reflects the risk and trust in the relationship.
A formal contract is a good way to establish "rules of engagement" (i.e. who does what for how much), and using it, enable enforcement of those rules as it is recognized under law. However contract enforcement is quite a different beast altogether. Although it's cheap to draw up a contract, it's expensive to enforce a contract using the law, even if you win, you still need to enforce the courts decisions..... Cross international boundaries, and the costs and complexity skyrockets, with little hope of any useful outcome.
As a rule, draw up a contract so you both have a clear understanding of whats expected, then put it in the bottom draw and forget about it, as needing to refer to it means the deal has gone bad. It's your very last resort when all else has failed.