Yeah, I've been doing that (quite often) I toned down on some skills and only used what was useful (aside from First Gem Costs and Mana per kill skills which are maxed(7 atm).
You can get some additional mana by downgrading your maxed skills to 6. Every third skill level gives an additional bonus, so most people say you should only increase in threes.
Also, see if you can get any achievements, they're worth 1 skillpoint each and some are really easy to get.