1. A stud finder.
2. A laser leveler.

I know in the dark ages before stud finders you'd just knock along the drywall to detect the stud. Oh, there's nothing better than pretty well knowing exactly where it is and when the drill goes in to make the pilot hole there's just a big happy smile when you hit wood.

I also picked up a cheapy laser leveler - just bubbles to help you determine if it's level before switching it on. Oh so well worth the money.