OK, I've had a few encounters with rattlers in SoCal.

First, if you hear the rattle, it doesn't sound like a baby rattle or anything. The closest sound I can equate it to are those automatic pop=up sprinklers that you see in places like golf courses. When they first pop up you hear the "ppsssssttt... pop, pop" as they start shooting water. Anyway, it sounded really similar. Enough so that I stopped and thought "Why is there a sprinkler here?!?" in the middle of a mountain range.

Second, they sometimes hang in groups. Before you retreat, check around you to make sure you don't step on his buddy.

If you're moving real fast, sometimes you'll cruise right over one. I saw one once, my dog went sprinting past me and kept going. I waited 5 minutes for the snake to get off the trail (and a couple of rock-carried suggestions) and couldn't call back my dog because I was afraid she'd get bit the second time around.

Cold mornings - they like to get near heat, ie, around your sleeping bag or tent. The nice thing is they're pretty sluggish in the morning. I stepped literally inches away from one, bent over trying to find some keys, and realized my face was about 2 feed from a rattler one morning. It didn't budge, but I sure did.

Have fun!