In roughly order of importance.
1) My saws have a raised plastic line and/or black line on the airfilter cover and starter cover.
(broken image removed)
Those lines need to be pointed towards my drop target while I am cutting the notch. If they are not, I am almost guaranteed not to hit my target.
2) On a 30" tree I'll use an open face notch and leave ~2.5" of hinge wood, so the tree will be parallel to the ground before the hinge breaks.
http://www.osha.gov/SLTC/etools/logging/manual/felling/cuts/notches.html
3) If the tree isn't clearly leaning the direction I want to go I make sure I have a tensioned rope up in the tree. The higher the better.
4) Your notch should be about 1/3 of the way into the tree. If its 1/4, it will be reluctant to fall. If it is 1/2, you risk the tree falling in the opposite direction.
5) The hinge should be sturdy and pretty darn close to level. If I make my notch cut and the groove isn't level, I'll take a little more off so it can be. When I make my back cut I stop and check several times to make sure I'm leaving the same amount of wood all the way across the tree. If I have a 2.5" wide level hinge 1/3 of the way into the tree like that, it will probably want to fall over on its own. If not I'll persuade it by pulling on the rope or knocking a felling wedge into the back cut. I will not cut the hinge wood smaller on a large tree. Cutting through your hinge on a big tree is how you end up on youtube with a flattened pickup, porch, etc.
Practice hitting a garbage can lid with 8" trees. Once you get that down, avoiding houses and sheds with 30" trees is the same thing + adrenaline.