Madden NFL 09 uses a Development Trait system (hidden initially, visible in Franchise editing) to govern how players improve week-to-week.
Each week, players earn XP (Experience Points) based on the above. XP is spent automatically to increase attributes toward their Potential ceiling (0-100 scale). Once Potential is reached, growth stops.
Maximize development by starting young players with high Dev Traits, scheming them into playtime, and hiring developmental coaches.