I look at my players for each playoff week and determine strengths and weaknesses based on abilities and match ups.
I look at my first week opponent's players (if known) and determine strengths and weaknesses.
I look at the waiver wire and determine if picking up any players would make my team stronger or my opponent's team less strong (if I take a player so he can't get them).
I determine if dropping a player would strengthen any playoff opponent's team.
I weigh the options and pick up a player if it will help my team, hurt my opponent's team and not result in a valuable player being made available. Sometimes this is a trade off and/or best guess situation.
This year, my opponent next week is weak in the RB spot. Chris Ivory was available on waivers. Although I have Hillis, Bradshaw, Tolbert and Westbrook, I picked up Ivory so my opponent cannot use him. Actually, I may play him this week based on match ups and his recent performance. I dropped Mike Williams (Seattle) because his injuries look like they will keep him out at least one game, maybe more. I'm not happy about dropping him and will keep a close eye on his progress. If I have any WR injuries or he heals fast I may need to pick him up again next week. Fun time of year!