Well, you can't do an official keeper league on Yahoo, but as long as you do an offline draft and input the rosters, you should be fine. ESPN does offer the opportunity to renew your league at the end of the year tho, so you might want to go that route.
As far as rules, it's pretty much the standard fair as far as scoringt and rosters go. The only real difference is you keep players(usually around 3) at the end of the year, and then draft from the remaining players at the start of the new season.
Hope that helps
16-team PPR Keeper League
QB: T. Romo, V. Young
RB: M. Turner, B. Wells, T. Hightower
WR: A. Johnson, J. Ford, N. Burleson, A. Roberts, J. Simpson
TE: D. Clark, J. Cook
K: R. Gould