I hope you're using a memory card for stuff like pictures, custom ringtones, music, etc. For more tips on memory check out How do I free up memory on my device? - BlackBerryFAQ
The 8300 suffered from "memory leak", a condition under which the device's free internal memory gets lower and lower with even basic use. The way I heard it explained, when you open an app it grabs a chunk of internal memory for use. When you close that app it doesn't always release that memory back to the device, instead hogging it and causing it to show up as memory in use. It's a limitation of the OS more than the device.
Upgrade to the latest OS for the 8300: http://www.blackberryforums.com/gene...ilanguage.html
Follow the instructions for upgrade found in the BB Faq: How do I upgrade the OS on my BlackBerry? - BlackBerryFAQ
Remember to delete the vendor.xml file as per the instructions!
Make sure to fully quit apps when not in use. Allowing them to idle in the background is a prime factor in this dreaded memory leak.
When free memory falls below 15 MB, do a quick battery pull. This forces the BB to do some internal housekeeping and to release that leaked memory.
This memory leak was cured (or at least very much improved) with the release of OS 4.6, but unfortunately that was never released for the 8300. The latest version of 4.5 for your device might help some and is worth a try.