Many factors come into my team selection-
Condition is a main factor. Tired players, won't last the match, they won't get into my starting 11. Its about resting them, in prep. for the next match.
Form- If your winning matches, and condition is fine, then why change something that isn't broken?
When things aren't going well- Rotation- Try different players and fresh ideas in the team, until you find the right team/tactic to get you winning matches.
I think the use of younger players is also vital, some younger players do feature in my first 11 on a regular basis (Ryan Leonard, Yala Bolasie, Joe Mason) and thats all fine, with a mix of some expierence. Obviously don't overload your team with Youth players!
Hope this helps 8-|