I only try and keep two teams at a time. That way I always have one really good team and one pretty good team as a fitness team.
The only exception to that is my Silver Belgian pro league team. I play them just for fun every once in a while.
Otherwise. I try to sell off as many players I can that I am not using so I can input more coins into whatever team I want to use. That said I still keep a hold of IF's if they are "goodish" ones as I tend to buy them when they come out and sell on later when generally their price goes up. I bought two IF Miccoli's when they first got released. Each one cost me about 50k. Now to buy him you are looking at ~150k

Might sell them on eventually, but he is a pretty awesome IF card so I might hold on to one in case I decide to make an Italian team or Serie A team again.