Hi, there're something that I can share about:
1. Since you have just bought your device for a few days, these two things might happen: a) the battery has not been fully activated, b) the battery meter has not been calibrate. I have experienced exactly the same issue like yours during the first few weeks, the battery holds a long time in "100%" and drop fast to "5%", the "5%" lasts for another period and then the battery ran dry. So, what I did is, USE IT LIKE NOTHING HAPPENED, charge the phone after the battery is nearly drained or drained. After a few times, the battery will be fully activated and the meter is calibrated as well.
2. Disable Background Applications. Go to setting -> applications, then block all background tasks. It might affect the using experience a bit, but at least certain IMs could still run in background's background LOL.
3. Remove Live Tiles such as "Me Tiles". The tiles with autoupdate functions will prevent the phone from idling.
4. Switch to 2G unless bandwidth is needed. although WP8 has pretty good power management in cellular networks, stay in 2G would keep the best cellular signal strength and prevent the basband chip itself wasting battery in searching the Base Station.
There are a few more tweaks for battery life but I think the above suggestions would be generally enough. I have bought my 925 for approx. 6 weeks and here are some figures FYI.
1. It could pick approx. 600 photo in one-charge (the screen was on for more than an hour), with a fair- poor cellular signal.
2. It could last more than 5 hours in web surfing with cell networks, for sure 2G will last longer than 3G.
3. Gaming. IDK cuz I dun play games.