The enchantments are completely random, all you have control over is the level you enchant at. The higher the level, the better the enchant is usually going to be. Some times you get **** on a level 50 enchantment.
I forgot its name, but there is a mod for custom level enchants. It as two arrows to change the level, and you can add enchantments and refund them. Maybe its worth getting it on your server, providing its vanilla (I think it as a vanilla SMP variant)