Keeper leagues are best among a group of friends.... If you can't gather a group like that that will be committed, try and find an existing league where a player is dropping out and inherit his team.
My Keeper league is Yahoo, and you can set the draft so that the first picks, the keepers, happen automically, and then the draft continues live like normal.... this way, the number of keepers needs to be an even # so that the last place team will have the first pick.
I'd also recommend not having too many keepers, otherwise, dynasties can be created that are difficult to topple for years... in our 12 team leage with a 22 player roster, we've settled on six keepers per year.
We've also made a change this year to help out the bottom of the pack teams. Once keepers are named, from last place to first, the player has the option to switch one of his keepers for one of the players that weren't named as a keeper by the other teams. This way, a last place team with a weak keeper list can quickly upgrade his keepers before the draft.
I also like the 12 team 22 player roster format, because it means there are usually decent players on the available list.