Game Shape is one of the most important things to your performence!
it will change from week to week, depends on the minutes your player gets
if he doesnt play enough (say less then 48 minutes a week) his GS will probably drop (not always)
if he playes too much (say more then 80-85 minutes a week) his GS will drop
basicly, you cant play your star players as starters in 3 games aweek, and you will need the 3rd game (cup or scrimmage) to give your backups some game time
you can train GS and it will effect it, but all other training regimes has nothing to do with game shape
about DMI, it's an index that is quite missleading
genneraly it tells you the odds of this player to give good performence and depends upon GS and skills
if your stars are less than 10k, then i assume they havre bad game shape, and will probably give bad peformence
but i wouldnt pay too much attention to DMI, you can play your stars still, just try to keep the minutes in the future