Tip #One: This tip works for pictures hung centered on a narrow wall and makes measuring easy. To find the center of the wall, use a cloth (or plastic) measuring tape to first determine the width of the wall. Now you'll want to know where the center is. If the wall is 20" wide, then it is a cinch to just divide that number in half. The center is at 10". But if the wall measures 23 and 3/8" it's not so easy to figure out what half is.
Instead of using brain power, just fold the cloth measuring tape in half and hold it, (still folded) up to the wall at the height you want to hang the picture and mark the center in pencil. That's where you'll put your picture hanger.
Tip #Two: If you are like me when putting a nail in the wall to hang a picture, etc., you may accidentally drive the it in a little too high or low or over too far one way. In that case you'll have to make a new hole in the wall and patch the hole that was positioned wrong.
The easiest and quickest way to fix that hole is to patch it immediately. In a pinch, I've used white toothpaste (paste kind), ivory bar soap and other substances. But my favorite product to use for patching nail holes is Patch and Paint Lightweight Spackling by Custom Building Products. I find it at Home Depot. It is very, very lightweight. In fact when you first pick it up, you will probably think that the container is empty.
This product can be applied with your finger (perhaps it would be best to use a plastic glove). It has only a slight odor (not at all unpleasant), drys quickly, (can be painted in 15 minutes), needs no sanding and cleans up easily. Keep a container of this in each room's closet, so you'll know just where it is when you need it.
Tip # Three: I've mentioned this tip before, but it bears repeating now since it is the next step in the process of "hang, patch, paint". Throughout your house, keep a small container (baby food jar works good) of the color of the wall paint for each room in that room's closet. That way you can pull out the paint from the closet and paint over the patch 15 minutes after having applied the spackling, without having to look in the garage forever for the right color paint.