When it comes to your beard there is no more dangerous time than trimming. It's one thing to mess up your own beard during a trim but it's significant worse when someone else does it i.e. your barber. Most bad beard trims from a barber stem from poor communication and not properly describing what you actually want in a beard trim or a mustache trim.
Learning how to properly trim your own beard and mustache is a very valuable skill. If you have to have your beard trimmed by a barber there are some tips and tricks to getting exactly what you want.
It's true in the saying that a picture is worth a thousand words. This applies perfectly for a trim. Bring in a picture or a few that will show your barber exactly what you want. You don't need a picture of a beard that incapsulates everything you want. You can point out aspects in the picture that you want done. Sideburns, lines, mustaches, and length, can all be pointed out. A picture will go a very long way instead of just trying to describe what you want.
The one phrase to stay away from is sitting down in a barber's chair and saying that you just want a trim. This will lead your beard trim up to the interpretation of your barber which could be a wildly different expectation than you have. The more direct and distinct you can be while also showing a picture/pictures the better off you and your beard will be.