It's a good way to make money if your time is flexible, and I've done my fair share of it, hopefully this rant/guide will help out anyone who's interested.
1. Don't expect anything to come from it
2. Expect dates to change, a lot. Unfortunately extras are the last thing on anyone's mind.
3. The days/nights are long, usually 12hrs, but I've worked anywhere between 2hrs and 15hrs plus travel.
4. Take things to keep you entertained, books, magazines, music, sketchbooks, whatever takes your fancy. I don't like to take anything valuable, like a laptop, as you could be leaving your bags somewhere while you're on set.
5. Take snacks. You normally get breakfast when you arrive, then lunch 6hours later, then sandwiches 6 hours after that. It's a long time for me to wait between meals, so i eat little and often. (This especially applies when doing period dramas, if you're in a corset or tight clothes, you can't eat a big lunch, so you start getting hungry a lot quicker)
5a. Also, don't be greedy at meal times. Extras have a bad reputation, so don't fall into the trap of thinking, "It's like an all you can eat buffet!" Remember that you've had an easy day compared to the crew, they need more food than you after carrying all those cameras/lights/bits of set, so do you really need a lamb shank, a piece of lasagne and some curry, as well as a selection of deserts? Besides, you'll be regretting it later on.
6. There will be weirdos who try tell you their life story, it will inevitably end with them suddenly deciding they want to be an actor (and they think they've made it by working as an extra), and it will result in them reciting every film they've worked on.
6a. There's also the serious actor-types who fully submerge themselves in "their character".
On the last thing I worked on, my friend was placed with a man and a woman so they appeared as a family. The woman took it upon herself to start dealing out back stories, their names, how far they'd travelled to the ball etc. So my friend turns to her "father" laughing about how it's really is like an episode of Ricky Gervais' Extras, only to be told how offensive he thought that show was to true extras.
I have so many more points, too many for just one post, but I'll be back!