The other day I had to go get fitted for a tux and the guy asked me to take off my sweater so he could measure. So I told him that I needed to use his changing booth for a second. I handed it to my wife. Didn't want SWAT showing up at the mall. Would have loved to see the look on his face if I raised my shirt and exposed my gun.
Last time I got fitted, I just took off my fleece. The guy hesitated for a minute, until I said 'This is how I will be dressing so take your measurements.' No questions, no comments, and it fit great.