Short battery life on a brand new iPhone

Well I got my iPhone 4 about a week ago. When I got it I did several full charging cycles, 3-5 if I remember correctly. And these days an iPhone battery really seemed "unkillable". It was really hard to drain about 50% with wifi and 3g internet, games, photos and videos, music, and several different apps.

But now I experience something strange: my battery drops 30% by night (just locked screen), and when I use my iPhone not so hard as first days (just music and Safari), battery has only 30% remaining (out of 100%) in 5 hrs.

Well, it's very bad. I don't want to charge my iPhone twice a day.

Also, when I leave my iPhone charging for night (I know it can fully charge in 1 hr), I see 93% in the morning. And it doesn't increase.

So, as I said I calibrated my battery several times when got an iPhone, and now battery drains super-fast. Can anybody tell me what's wrong and what should I do to make my battery work as expected (at least 10-15 hrs on hard usage, but 20+ hrs will be welcome 😀)?

Hope to hear the solution. I tried setting it up as new already :/

(The same question is on StackExchange too http://apple.stackexchange.com/questions/28094/short-battery-life-on-a-brand-new -iphone)