Now, I will admit that I am not as crazy about keeper leagues as I am other formats, it does have its place.
Keeper leagues are similar to dynasty leagues in that you retain players year over year. The difference is, with keeper leagues, you only keep a select number of players. That number changes league to league. Some leagues assign a dollar amount to players and it is up to you to decided if you want to continue to roster them or throw them back in the pool to bid on again.
On the surface, this would seem fairly straight forward. Keep the maximum number of the best players you have on the roster. When you look closer however, there is some gamesmanship involved here. True, plenty of choices will be straightforward, but here we can get into the topic of positional scarcity.
By definition positional scarcity is when there is a lack of talent at a position or the gap between one tier and another is so great, that it forces you to take or retain a player that you might otherwise wait on. For example, at the tight end position, there is a big three (four if you count rookie Kyle Pitts). Outside of those, there is a significant drop-off to the next grouping. If your league only allows you to keep three players, for example, and you roster Travis Kelce, Darren Waller, or George Kittle, they should absolutely be on your keeper list.
The definition of these types of leagues is simple, but often gameplay is quite complex. Each player is assigned a value, whether that be monetary or just a points-based system. Generally, you are assigned a cap that you cannot exceed.
From there, you have to determine whether keeping player X at Y units is worth it or weigh that against collecting another player with potentially higher upside. Often times, these are combined with year restrictions (see below) to increase the difficulty even more.
I would strongly advise using some type of document or spreadsheet to track your players and their values if you are going to play in these types of leagues. You need to be able to determine where you stand when it comes time to compare a player against one you may get in trade or on the wire. This also allows for budgeting and planning when free agency hits.
Similar to value leagues above, some leagues count years instead of monetary units. On top of that, many leagues will have a rule that you can only retain a player for X number of years and then are forced to throw that person back into the draft pool. This is also something to consider when trading for a player, you will need to know how many years you have remaining and, if the league allows, how many more you can extend him for.
Also, like value leagues, I would highly encourage you to keep track of your team’s years in some kind of spreadsheet. This is particularly handy if your league has a cap on the number of years you can have or penalties for going over a limit.