What people (how old are they, what is their education, where do they live, what is their profession, their ethics system, the state of their health ...) and for what purpose?
Fear! It has worked for centuries! haha in all seriousness though, different things motivate different people probably the easiest thing to do is ask the party what they want, and offer that as a reward for a said action
Greed. Greed has motivated humans since the beginning of time ...and it's not likely to change in any foreseen future. Baron Max
As Robert Townsend put it, in his book "Up the Organization", you can't usually motivate people. That door's locked from the inside. What you can do is set things up so they motivate themselves.
