I was working on a piece about how to write a bio, but I doubt I'll finish it for a while now. I think the main point about press kits is that they are marketing, pure and simple. Your band (or for live gigs, your show) is the product, and you're trying to sell it.
That said, there are a lot of approaches to how to sell it. I've seen people do what pashop suggested and it can be pretty slick if you take your time and design something nice.
When I was touring we usually had a packet that had about 4-6 items in it:
1. cover letter
2. bio
3. 8x10 glossy of the band
4. Endorsement page (i.e. a page full of quotes about the band from other club owners, music critics, well-known musicians or celebrities, etc).
Some venues asked for certain things like lyric sheets, tour itinerary, etc. Keep in mind this was all in the early days of the internet, so we couldn't count on people being online and going to our website for more information. We had to give it all at once. That's why we broke it up into several pages instead of cramming it all on one page. They could read what they wanted and leave the rest.
The key is to remember that every aspect needs to appeal to the people you're trying to reach and communicate what your band is about. A slick glossy pamphlet might turn off the guys who run the local underground venues, but it might work better for some upscale clubs or corporate events. Smiling glamour shots of the band members is probably a bad idea for a hardcore metal band.
Anyway, there are books on the subject that do a better job than me, so I shut up now.