Instead of when you cast a spell that is already on you and it says that that spell is already in affect...have it instead reapply the spell bringing it back to full duration so that you can maintain your spells. I believe it already lags you and takes the mana anyway when you do that, might as well go the rest of the way.
I coded this for another mud and found it easy to implement(not that the coding is what holds back new ideas, at least I hope not) and went over very well.
