1. Make sure you have a bunch of friends that play the game, or make some. The more, the better.
2. Don't bother playing missions all the time, just do a few every once in a while. Know which ones to pick that you can earn off of. Stay away from the missions that Rockstar has made pointless with their reward cuts.
3. Explore. Drive around. Get chased by the cops. Do everything that you would to f*ck around in single player, but include your friends.
4. Come up with new little mini games. Challenge your friends.
5. And finally: Don't take things too seriously. GTA is meant for you to have retarded fun. Use your imagination.