Player boosts improve player strength for a certain amount of days.
Each player can only have one boost at a time, so if you boost a boosted player, the old boost is replaced. Only players who reached their strength maximum (no + shown after strength) can have them.
These are the existing Boosts:
Silver-Boost: +10 but only available for players with strength max 59. Available once daily in the team office (but at maximum 30 times), and for 11 orange collectibles until level 24.
Shoes:: +1. Available for 11 orance collectibles from level 25.
Girlfriend: +2. Available as reward for a special challenge. You can get that challenge only if you have no "Single" girlfriend.
Protein: +4. Available in platinum treasures.
Worldstar: +7, decreasing by 1 each season change (sunday night). Comes only "preinstalled" on certain players in certain packs.
Halloween: +6, but only for superstitious players, and only available during halloween specials.
Enlightenment: +10, but only for superstitious players, available in platinum treasures.
Jester Hat: +2, only available during carnival specials.
Summer Boost: +2, only available during specials.
Extra Training: +1..+4, only created by a special coach