Make the game harder. So adjust the formulas/algorithms you use to make leveling/training etc harder.
Put a cap on the attacking level. so you have to be in a certain level boundary to attack certain players. 1 => 5, 6 => 10, 10 => 18. You want to make it so that the other player doesn't get completely destroyed.
Don't make OP weapons or armors. That just ruins the game. Even if they cost a ridiculous amount. If you're going to have those sort of weapons have them on DP (donator packs) or a way which anyone can them.
Those a few I thought off, I'll add more as I think of them.