I've never played in a keeper league, but I think it would be interesting. There are a bunch of different variations, but the version I've heard most frequently used generally follows the set of rules below:

- Year 1: everybody drafts and plays the year as normal (trades, free agents, etc...)

- Start of Year 2: Each team declares three players they want to keep for the next year. You draft the players that you want to keep in the draft slot you chose them in Year 1 (e.g. if you drafted Ryan Braun in Round 10 in Year 1, your Round 10 pick in Year 2 will be Braun as well). If you decide to keep a free agent pickup from Year 1, you give up a late round pick. I don't really know what round you give up if you decide to keep someone you traded for.

- Year 2: Play as normal.

- Start of Year 3: Same "keeper" process except if you keep a player for the 2nd year in a row, his draft spot will be the Year 1 draft slot divided by 2 (e.g. if you decide to keep Braun again, you'll have to pick him in the 5th round).

- Year 3: Play as normal.

I'd be interested in a keeper league. The difficulty with doing it on the GBB is the turnover involved.

Any interest in making this year 1 of a keeper league?