Your barber gives you a lot of control. He'll do anything you want, but you've got to tell him to do it. And when you don't make your point clearly, you could get out of that chair a very disappointed man. If you don't tell him exactly what you want, how's he supposed to know? The first thing you're asked when you sit down is usually some form of "What are we doing today?" And a response like "just a trim," isn't doing anyone any favors. Some guys can be intimidated by the lingo, so here's a handy cheat sheet on what to say the next time you sit down in the barber chair. Nothing radical or earthshaking—simply a few key instructions that will make a big difference.
Try to provide the barber with a general description of what you're looking for. A "clean up" that will keep the same style you currently have? Do you want an old school style with a part or something more modern and casual? Once you've got the barber on board with your expectations, you can move onto the specifics.
Every haircut is different and so is every barber. Terms like short and long are all relative and sure to lead to misunderstandings. Specificity is important when you're talking about how much hair should be cut. Familiarize yourself with what an inch of hair looks like so you can confidently say things like "an inch off the top" or "a quarter inch off the side." If you're not sure how much you want taken off, tell him that and ask him to show you how much he's cutting.
Some barbers will ask you how you'd like your nape—where the hairline meets the back of the neck—cut. Some offer "squared off" or "rounded," but a lot of barbers have their own preference and simply trim it without asking. Make sure to specify how you'd like it done. A square, blocked nape can add the illusion of width to a slim, long neck while a rounded nape suits thicker necks. But in general, a tapered nape (clippered and faded into the back of your neck) tends to look a bit neater, more natural and grows in better.
This is where you specify how short or long you'd like your sideburns and the kind of texture you want cut into the top. Don't skip on this part. If you've got thin or thinning hair, make sure to use words like "choppy" and "layered," which means the hairs are all cut at different lengths, resulting in a thicker appearance. If you've got thicker hair, ask for it to be "razored" or "thinned out" to take out some of the bulk.