This you do by managing your minutes per week. I cannot give you exact numbers, just the ones that work for me.
When you consistantly play a player the whole match, 2 matches á 48minutes, thats 96 minutes for the player. Sometimes you can even let him play the scrimage, another 48 minutes. This is bound to destroy your Game Shape.
From my own experience, players who consistantly play 48 minutes per week (so 1 full match), will grow to Game Shape "strong" (level 8) at medium to high sublevel. Once you start playing a player 55-65 minutes continuesly, his GS will grow to proficient (level 9 which doesn't have sublevels). From 66-75 minutes you will get into the higher sublevels of strong. Any more then 75 or any less then 48 will eventually drop your player's GS.
A second option is training GS, but this is not very advisable. Especially now with the season changing, and the Game Shape to be reset soon.