Agree with Hejst on most points. My experience is that in general, the use of wingers and players in between lines (attacing midfielders and defending midfielders) perform well most of the time.
Also, don't try to play a player in striker, unless this was the players original role! I have tried amc's, amr's and aml's to play as striker and almost all of them performed bad.
At last, watchs ads for rest points. You can do something else in the meantime, but it will earn you up to about 20 rests a day. Good luck!