1) A link on your site that you have to login with one of the selected accounts to use (prbly best option)
2) Coupon on Smashwords
3) A link to a dropbox, SkyDrive or other free file host that you email out
4) Get snail mail address and and email cd's with your collections on them (hey it IS an option...
5) A password protected link, and then email out that password and link to your selected audience.
Thats at least what I think of off top of my head. I like #1 best because it could discourage people form posting the link some place, while any of the other methods people could do that extremely easily.