Why does my iPhone 4 run out of battery so fast?

I have an iPhone 4 with only 18 pictures and 28 videos. I have 10 apps aside from the apps that already come with the phone. I constantly delete pictures, videos, and apps that I don't want to delete just because of storage and it still runs out shortly after! Or sometimes it doesn't make a difference after I delete something! Why does it only let me have so little?

Storage shouldn't affect battery life, there are many ways to extend an iPhone's battery life:
Close apps in the multitasking bar (double press the home button)
Don't leave Wi-Fi searching for networks, turn it off
Turn off push notifications and push emails
Turn off location services
Turn off spotlight search
Turn off 3G
Basically make sure you aren't using power where it isn't needed.