I just got my first beard trim from a barber and while it was fresh on my mind, I wanted to share my thoughts on trimming. There are two ways to trim your beard - do it yourself, or go to someone to do it. When you trim yourself, I've always been a big fan of using scissors instead of clippers. I've heard many of stories where beardsmen lost their beards because they blindly started trimming without checking the guard. When you use scissors, you don't have that issue.
Your beard will grow at about 1/2" per month, so if you want to maintain your length, you'll want to cut that much off every month. Perhaps every two months you can trim an inch off. Comes down to your preferences. For a good beard trim, less is more and it's best not to do it when you are rushed. Go in a shape that follows your natural beard shape and only cut off a little bit. I've found that trimming my beard over a couple of days works best.
If you are going to trim your beard with a barber it's best to find one who does it regularly. Generally speaking, if they have beard trim on their services list; they probably do a fair amount of trimming. If you aren't comfortable with that, then go to your local beard club or ask your local beard club president which barbers he'd recommend for a trim. Chances are they are well connected.
Hope this helps and keep those beards looking neat!