I don't keep the same formation every game. I keep the players to switch to a back 3 with or without WB's, always have a good 2nd CM and DCM, have the players to play any of the 4 levels on the wings, good attacking mids, and can go with 1-3 ST without losing quality.
I know a lot of good players here keep very little in the way of a bench and play the same every game but I just like to go by matchup. I get versatile players so that I don't have a ton of guys moving in and out, that way they stay in form and don't lose morale.
I have been fairly lucky that I haven't had many dud signings. Took quite a while to get my main DCM going and the jury is out on one fairly new wing player but most all of them have been consistently above a 7 avg. My Strikers have been excellent (44G 20A between the main 2 in 30/25 games) My two wide MF are my FK and CK specialists and have contributed to around 20G each and my GK allowed all of 2G in the last 17 league games though the cup games were more adventurous.