Very few times in my life have I ever felt the need to clear my weapon in a commercial environment. If I'm in a gunshop and find a holster I might like, I'll either ask to see one of their 1911's, and check for fit (how much of a sweat guard it has, covers or doesn't cover the mag release, etc.). If that won't solve my curiosity, I'll either:
1) Tell one of the staff members what my intentions are and ask how THEY would like me to proceed. This puts the ball in their court, and lets them operate under a condition of control. Having been on the other side of the counter, we appreciate that. Often times I'll just hand my gun (still in it's holster) to them, after telling them its "cocked and locked, with a round chambered" and let them clear it.
2) Go out to my vehicle and come back in, carrying my UNLOADED weapon in it's holster in my hand. I'll walk straight to a staff member and hand it to them, so that they can verify that it's cleared, and what my intentions are for it.
I can't tell you how many times I've tried to stop someone in the middle of a draw of a LOADED handgun in the middle of my (then) crowded store, only for them to pull the mag out quickly, rack the slide and PULL THE TRIGGER, before I can get my hand on it. I've also had occasion to have a half-dozen or so loaded handguns pointed at my midsection, because people don't have enough muzzle control while they are trying to clear their gun.
If you're in your car, draw as normal and point it straight towards the ground, between your feet, and clear. Don't let someone see you do it, either. It has "warranted alarm" from me in the past.