Focus on robbing the Jewelry Store and Bank back-to-back. High-efficiency players often use air vehicles for fast travel; these can be customized at the airport hangers.
If you prefer to avoid the risks of script-based farming (such as account bans), these are the most effective manual strategies: Mad City XP Auto-Farm
Running back and forth while working out in the prison gym is cited as the fastest "zero-risk" way to reach Level 100, especially when done in low-population servers. Focus on robbing the Jewelry Store and Bank back-to-back
Teleports the player between high-value locations like the Bank, Jewelry Store, and Casino, immediately bypassing the "robbery" wait time. immediately bypassing the "robbery" wait time.