Keeping Boquets From Wilting

I am going to have friends make my wedding boquets. They are going to be very simple tied boquets of roses and wax-flowers. I wanted to know what I should do with them to keep the roses from wilting and how far in advance I could make the arrangements.

Here is a flower tip for others out there. We are getting our roses from Costco for $12.99 for two dozen roses. I haven`t found a better deal than that.