These tips are from The Tightwad Gazette.
1.) Cut it dry.
2.) Don't cut it too short. That's a sure sign of a home cut.
3.) Decide the style before you start. A bowl cut will require a different strategy than a short "barber cut".
The next items are for boys....I also cut my daughter's hair, but it's not quite the same.
4.) The stylists I watch usually start at the top of a boy's head, halfway back, hold up a thin section of hair between two fingers of the left hand so that the scissors can use the upper edges of the fingers as a guide, and cut. They then gather the next section, still at the top of the head but a little further to the back, and use the previously cut section to guide the next cut. They work their way back until the back of the head is reached.
5.) They check by combing up a section going in the other direction, picking up a section that resembles a tomahawk. They comb that section up and check for a straight cut, and they like for it to match the curve of the head.
6.) Towards the front, they use the same procedure, but you have to take into account what the boy wants the front to look like.
7.) The sides are cut by holding vertical sections straight out to the sides, and cutting them with a vertical cut, angled slightly to make the hairs closer to the chin gradually shorter than the ones above them. The hairs lie like shingles.
8.) The back is cut like the sides.
9.) The neckline, sideburns are shaved with an electric haircutting tool or whatever you prefer.
10.) An electric haircutting tool is great for doing sides and back too, as you can switch out the attachments as you go to whittle down the length gradually.