Yeah, what he said, except that I always duck. It's instinctive, and I figure that there's a good reason for that particular instinct, so why fight it?
Well I'm short so I'm good.

Seriously though I actually look up at the blades as I pass under them. If it's at night, sometimes I'll shine my light up there to ensure I have clearance. Biggest thing is just make sure the PIC is guarding the controls and knows you're approaching.
I came within about a foot of decapitating a French Soldier in Afghanistan. We were doing a resupply to a pinnacle and we were only able to put the mains on with the tail up in the air. The guy came running up to the aircraft from the worst possible position; right front quarter. That's exactly where I had the cyclic displaced to counter the slope. My CP in the right seat yells "S--t! Watch this guy!" I pulled just enough aft stick to clear his head but when I did we slid backwards and our right wheel snagged the crew chiefs cable pulling it from his helmet. He lays on the ground next to the tire and says "Sir you're on my cable, pick up your wheel so I can get it out." All of this at 8,000 ft, on a ridge with 35 kt winds blowing us in the middle of a combat op. Good times.
Suggestions for those going on their first helo flight:
1. Approach the aircraft only after one of the crew members wave you in.
2. Approach the aircraft at the sides or slightly aft of 3 & 9 o'clock.
3. Avoid the nose and tail. Tail is obvious, nose because most (all the ones I've flown) have a forward tilt in the transmission so it's the lowest place on the disk.
4. If you happen to be in the unfortunate position of being in a crash, resist the temptation to immediately evacuate. Unless you're on fire stay put til the rotor blades stop. If you must exit go to the sides, unless it's sloped, then just go to the lowest terrain.
5. Don't wear a hat or any clothing not securly attached to the body while under a spinning rotor.
6. Enjoy the ride.