How to carry lenses

If you're shooting an event of some sort, how do you carry a couple of lenses (or one) on you? The camera is of course in your hand.

I normally carry all of my things in my Lowepro backpack, but I don't want to do that at event because if I need to switch lenses it is too cumbersome to get into the backpack and swap lenses. Not to mention I would need to put the backpack down.
I am thinking I could just use my Crumpler 5MDH to hold 1-2 lenses in it, but was looking for other suggestions?

Photo Vest.

I use a Domke and it's one of the best things I've ever bought.

Belt systems work great for events. I use a Think Tank Change Up beltpack with two lens cases that attach to the belt. It lets me carry three lens with one spot empty when I am carrying the camera in my hand. That way, I can detach a lens from the body, put it in the empty space on the belt system, take a second lens out of the belt system and attach it to the camera. I never have to juggle two lenses with one hand and I can change lens fast.

Vest or a belt system are 2 solutions. I don't use either because usually I am also at the event to enjoy it and do not want to walk around with a belt system. Also geeky looking

I had a Fastpack and found the same issues you have with a backpack. I got rid of it.
I have a tiny shoulder bag which holds 2 short/medium lenses.

Normally I go with a Lowepro ProMag2 shoulder bag. Holds 4 lenses vertically, a 580EXII, and a camera with mounted lens (up to 70-200 F4IS). No fumbling, no having to put it down and it's well padded. If I need to bring the 300, I attach the case to the Lowepro on the outside.

If you need fast access to safely and quickly change a lens then forget anything that goes on your back other than, maybe, a Flipside. The problem with that is it's not that "fast" and you need space to use it so forget crowds.

Generally I will wear a vest unless the weather conditions are too hot. When a vest is not practical I will take a small shoulder bag, my Tamrac 603. If the shoot requires my Sigma 150-500mm I will pack my Tamrac Velocity 9X which will hold that lens mounted to a gripped T1i plus a couple of smaller lenses and a flash. The Velocity provides easy access without needing to remove the bag.

Not too cumbersome if you use the Thinktank Rotation 360. I keep two lenses in the lower compartment which you can rotate to the front for access without removing the pack. Expensive, but you pay for accessibililty.

When you say event, do you mean socially, or you are getting paid? I can't imagine wearing the Rotation 360 tvphotog recommended unless it was something you were hired to photograph!

When I go to social events, usually I bring two primes, and a small adorama sling they gave to me for free. I keep it next to my girlfriends purse. Usually it's only one prime in there. And yes, I look back ever so often, even when I'm drinking.

I prefer the Vest as the option of choice. I have the Tilley Photography vest.

There are vests available in Black for those formal occassions...

Think Tank belt system (skin one) is really very efficient.
You also can consider things like Shootsac.com

My new take-everywhere event, parade, festival, family gathering bag is a Domke F6. I got it recently and wish I had picked one up much sooner. Perfect. After doing some minor surgery to the standard 4 compartment insert and with the addition of a 1 compartment insert (also with some mods), I can comfortably fit my 1D3, 24-70 with hood reversed and 135 with hood reversed. Those are the two lenses I like to carry to such events. The front pocket fits a flash with room for much more. Easy to carry and work out of. Comfortable and light, especially when the camera is out of the bag and in use. It sort of confirms to your body and disappears compared to the other bag I used to try and lug around. It was my fully loaded with EVERYTHING Lowpro Nova 5 AW. It weighed a ton, and after using the Domke, feels too heavy and stiff as just the bag. Over padded too. I think I'm going to get another, larger, Domke to replace it. Nice bags.