This article contains some great information.
How to Travel Grey
It’s very good advice for travelers, even those of you who only travel domestically. Lose the tactical pants, Molle-covered backpack, and ballcap advertising your favorite gun company. When I fly, I wear a loose fitting T-shirt and jeans. My carry-on is an Arcteryx Blade backpack that looks like something a business traveler would carry.
The author’s comments about the beard are right on as well. Several years ago I remember being constantly asked if I was a cop by immigration and customs officials when traveling. I think it was the fact that I was big, clean shaven, and had a very short haircut. I decided to grow a beard during an exceptionally long trip. It didn’t work.
I still got lots of attention from the local cops and immigration officials. When I came home, my then girlfriend’s teenage son asked me about the beard. I explained how I was trying to avoid looking like a cop, but somehow it didn’t work. He said “Of course it didn’t work. Now you don’t look like a cop. You look like a terrorist!”
Be aware of what your appearance says about you in other countries.