Trimming a beard is VERY different to having a hair cut, and whilst a lot of barbers will do an excellent job a lot can do there own thing - trim the beard too hard - bring the neckline up too high - or just not give you what you want.
So, here is the Beardsmith essential barber shop beard trim guide....
1) Treat the beard trim as it's own part of the visit - not just something that is quickly done after the haircut. The beard looking right is important to you! (This also means that you should expect to pay extra for the beard trim!!)
2) Take time at the start to explain in detail EXACTLY what you want, and DON'T want. They are not psychic and require your guidance. Include neck line, cheek line and moustache size.
3) Explain what your end goal is - what kind of beard style do you want?
4) Mention any weaknesses your beard has. For example if your cheeks are weak tell them this so that they can trim less in that area.
5) Ask them to be conservative - take less than they think and then go again if YOU want it shorter.
Take all these points into consideration when visiting the barber and you will end up with what you want from your beard. Really let them know that this is important to you and you don't want a standard one size buzzed trim of the beard. Or, just keep growing that beast out and give Gandalf a run for his money!!