For the girls I just have to trim their bangs regularly which works best to do with dry hair and start longer than you want and then work at evening it out. (this is a place where it is easy to get to short!) Every so often I will trim the back of the hair too, which I usually do with wet hair.
For mine we did something different. I figured that a layered cut is hair cut shorter on the layers of hair that are on top. If you have long hair and you hang your head upside down the hair from the top of your head goes the farthest down and the hair from the bottom is the shortest, etc. When my youngest sister was over the other day I had her cut my hair this way. I lay on the table with my head hanging down, as pictured, then she trimmed a couple of inches off of the end. The other pictures are the result. I think it worked pretty well. It is probably not the best haircut I have ever had but it certainly isn't the worst either. It is very fast and easy too.
If you would like more info on cutting hair here are some websites that have info: