One of these is referred to as T&M (time and materials). You charge a fair fee per hour for your time and expect 1 for 1 repay for cost of materials that you expend to do the work.
Do not forget to add 0.01 - 0.1% (= paid off in 5 - 0.5 years) of the cost of your tools (compilers, scopes, whatever) per hour. As a consultant you can not expect one job to pay $3000 for a needed compiler, but you need to recover the money eventually.

Another method Michael does not mention, which can be extermely dangerous and extremely profitable (I know a guy that made $250.000 for 2 months work) is pay per unit made. If you go for that one (be VERY careful) you are, in effect, investing in the success of the project.

Erik