there are a bunch of great usb charging devices out there, most under $50. A friend brought a few different types last year and the general consensus was the anything 10000mAh or better will keep your phone happy all day long.
I've said it before I'll say it again, sit in the front (get there early) if it's a topic you are interested in. It helps to take notes as sometimes these dim lit rooms make people a little sleepy (esp. after a good night out). Writing the notes keeps you engaged with the topic, and sitting in front will encourage you not to nod off as some speakers take that personally
I usually do two shirts per day, a tshirt (get it from the vendor area) and a longer sleeve over the top. That way you can have an extra shirt for the freezing rooms, and you can ditch it into your backpack when you go outside.
Bring at least one nice shirt, I didn't last year and regretted it. There might be an event that you attend that you may not want to look like a conference refugee at. (MS Community party for one)
HYDRATE : the water bottle is key, keep using it all day long, you'll be surprised at how much better you will feel drinking tons of water as opposed to tons of soft drinks (even if it is Diet Dr Pepper) or coffee. Keep that bottle full at all times...AND...know where the closest bathroom is.
When the room schedule comes out, look at it and compare it to what you have signed up for. You really don't want to go end to end to end between each session, it's exhausting and tends to get you the crappiest seat.
In addition to not breaking in your shoes on the day of the event (walking backwards apparently increases the break in rate, or so @rickstercdn tells me) carry and extra pair of socks. A change of socks half way through the day maybe the best thing ever
don't be afraid to interact. There are great BoF sessions to get engaged and social groups such as #theKrewe, use them so that you have someone to get ideas from, hang out with, and generally help you out if the need arises.
Ok, I've added about $1.25 as opposed $.02