Bonuses are a good way.
The amount of balance required is in the Help.... they are, in general:
For an N-year contract, no more than 1/(N-1) of the contract can be in any one year.
EX: A 3 year contract, no more than 50% of the total contract value in any one year.
A 4 year contract, no more than 33% of the total contract value in any one year.
A 5 year contract, no more than 25% of the total contract value in any one year.