I print out my cache pages and carry them on a clipboard, so if there are muggles in the area I just take the clipboard with me and play the "inspector" routine. I figured this out after sitting in a Wal-Mart parking lot for 15 minutes with a constant flow of muggles. I grabbed the cache, then moved on around the parking lot, making occasional "notes" and at the furthest one I signed the log. By that time the muggles had cycled so I returned and inspected the pole by my car one more time and drove off.
Another trick is to take a camera. I'm not a small guy so I don't have a lot of flexibility. Sometimes I'll point the camera under the bench or inside the low pipe and take a picture, then look at it. Several times I've located a cache that way. If anyone were to stop and question me, I'm just taking a photography class and our assignment is to take interesting pictures of normal objects and that the professor even mentioned the bottoms of benches!
I have known of people using the orange vest and hard hat tactic.
I've read several books where someone gets around a hospital, plant, office complex, etc. by just carrying around a clipboard and nobody questions their presence. It isn't just to act like you belong, you have to "look" like you belong too.
I was hunting a downtown Orlando cache not long ago, and people would look at me and then look away. Then I had a nice lady offer me a ride to the Salvation Army for a shower, clean clothes and a hot meal. I realized that I definitely looked like one of the transients that haunt downtown so that explains the behavior of both her and the muggles that looked away from me. Of course I had just completed a cache called "Fear Factor Urban Wetlands" and nobody can come out of that one looking clean, so I can't blame them.