The most effective way is to go to places with a lot of people and keep punching.
What I did was create a custom deathmatch map, then create npc with spawn points as close to each other as possible. Lock weapons to jerry can, and just go ahead and go test your map (solo, but make sure your character is your GTA online character, not storyline one. Ensure this by entering Creator from Online instead of Story Mode).
Start to switch to melee and whack the hell out of the npcs (they attack back). If you kill about 50-75 of them (takes 5 to 10 mins), i think you should up 1-1.5 bars. I can't recall exactly. After that you go back online and enjoy your improved strength stats.