Know-How Exchange : Strategy

Question for the professionals:

I have a start up magazine and I am currently trying to come up with a
sales rep pay structure. They are all outside sales reps. They visit
prospects, introduce the magazine and drop off media kits. I have read
alot on the different ways to pay sales reps but I'm still not sure which
is the best way to go in my case. I was planning on a base + commission
for the frist year and then the second year - draw against commission
every year after that. Should I pay expenses, such as milage? They are all
local reps. I have no way of really knowing what type of sales they will
bring in? Is there a way to figure that out so I don't overpay them in
commissions +salary?

Thank You