Cleaning things up.
There isn't really that much to clean up as compared to your computer. Most of this would depend on yourself than on to run some automated tool, some examples:
- Clear the history of the Internet Explorer.
- Get rid of older mails which you will never act on again.
- Uninstall applications you once installed but actually never need.
- Get rid of music, videos and photos that you tend to skip, because you don't wan't them anyway.
And if you want to get rid of any actual caching crap, you could consider resetting your phone each year and starting over. But I doubt if that's going to differ that much.
Does fragmentation hurt here?
Fragmentation is something known to be a bit noticeable on hard drives, but you won't really see this on your phone as it doesn't have a hard drive. Instead the phone uses what is called Flash Memory (think of a USB thumb drive, or SD card for your camera). Fragmentation isn't anything you need to worry about with this type of drive.
To summarize what you could do:
Depending on your usage, do a weekly or monthly cleanup of history and older crap.
Do a yearly reset of your phone if you feel that it generated crap, fragmentation or other slug.