Increase your Mind Magic skill to drain more mana. If you're standing right in front the monster, use Drain mana touch instead of Drain mana, it'll earn you about twice as much mana.
You can also buy and use scrolls of these spells, they'll cost you no mana (only money
).
I'm not really sure whether your (or the monsters') magic resistance affects the amount of mana drained or not, but you can try leveling up your Intelligence and Personality attributes if you want enemy spells to have less impact on you.
As for how it works on weapons, I have no clue. But enchaning them to drain mana can do you no harm.