Ok, my band's been taking press shots for 6 years, we've done magazine spreads, online features and a ton of other promo and still have no idea how to do this. It is harder than writing a good record in my opinion haha. Not to sound narcissistic, but all four of us are decent looking guys, that was never the problem, its everything else. Here is what we figured out after many many many failed and dumb looking photo shoots.
1. It has to be natural, the photographer should bring some music that reflects the mood the shoot is supposed to capture and play it the whole time. The ready, pose routine will usually end up looking weird.
2. Talk to each other before you dress for it. Don't all go to your closet and put on the thing that you believe you look the best in. We've done things where we all went shopping together specifically for a shoot. A couple times we were lucky enough to have a wardrobe person come with us, but those times are few and far between. Buy stuff and return it! Keep the tags on. Also, dont wear a thick jacket and a scarf and have the guy next to you in a sleeveless shirt. (No offense to the post above)
3. Do not wear make up, unless thats a big part of your band, which in that case sucks for you. A photographer, or even a 3rd grader at this point, can edit anything you dont like out of the picture, whiten your teeth, smoothen your skin, etc. If you wear make up then you're going to look like you wore make up.
4. Back to photoshop. You can take individual pics and splice them together to make a group picture. You wont believe how often that is done. We did a shoot once where we all liked a certain shot and our guitar player insisted he made a stupid face, so the photographer took his head from a different pic and pasted it into the one we all liked.
5. Look at other bands press shots, again no offense, but not on here. I'm talking about Coldplay, The kooks, Muse, Kings of Leon, etc. Those bands I listed have great press shots, especially the Kooks. Look at the pic, what they are wearing, the attitude most importantly, and the natural feel of the shots.
Those are all general guidelines i think could work for most bands. My band, has a ton more ideas and opinions on press shots and shoots in general but those are a reflection of our band and will not work for everyone. Start with that and good luck.