Don't rush your demo, that's the number one thing. Make sure you get a good studio and spend a lot of time getting good sounds. There are so many bands out there now you need to stand above all and show that you're worth their time/money. Choose your 3 or 4 best songs which all combined together show the overall style of your band (with the first song being the best/catchiest/most accessible etc, most labels only listen to the first song or the first few minutes before deciding if they're going to continue). Also this may seem pretty obvious but when you're recording make sure you're in perfect time, if a label hears a band that goes out of time in the song they're listening to even if it's small they may not think you're good enough musicians and might discard you right there; with protools these days its really easy to do click track stuff so try that. Your first song may be amazing but if they hear you falter or have a timing problem you might get discarded instantly.
If you can get some artwork, that'll set your cd apart from just a regular burnt cd-r that they'll throw in the trash and BE SURE to include your contact information (this may seem obvious but do in fact use cds in jewelcases with art, don't send an actual tape haha). If they like it but there's no email/phone/myspace or anything how are they going to contact you? A bio or big shows you've played and any credentials to show you're the real deal are good but don't include every piece of info and overload, just the really important and impressive stuff.
Pretty simple stuff really, just include the best/most diverse recording you can recorded well with some art, big important stuff your band has done in the past, maybe a picture and sufficient contact info. Hope this helps!
