I have a fairly large list of things you can do. I just tried to help my buddy get rid of 9.6 GB of other data. Sadly none of it helped. Hopefully something will help you.
By standard having a jailbroken iPhone will take up anywhere from 500MB to 2GB of "other data"
First off we will start with the basic things.
Delete the cache, history, and cookies on any browser you have on the phone.
Removing apps that you don't use or play anymore will also help. I believe game saves and settings in the apps also counts as other, but I could be wrong.
Delete any trash or junk email. Sometimes it can build up, usually the iphone will delete it after a certain amount of days, but its always good to double check.
Delete any tweaks or themes you don't use anymore in Cydia.
Plug your iPhone into iTunes and uncheck open iTunes when iPhone is connected. This presumably makes iTunes double check your "other" data.
Now onto the more complicated stuff, you will need ifile, winscp, or anything like these to access the iPhones internal files. I will guide you through ifile since its what I used. I recommend holding "Power" and "Home" until your device turns off and turning it back on after everytime you delete something. Keep in mind that ifile by standard doesn't delete anything, when you click edit a trash can will show up on the bottom and you can empty the trash that way.
Delete anything in the following folders (unless noted otherwise):
var/mobile/library/logs look for a file called ilocalis.log, if its there delete it
var/mobile/library/application/support/containers/ Look if there is anything big in there. (the blue button on the right will tell you how big the folder is among other information)
Okay now onto the next part: we can try to locate just one giant file that could be taking up all of your memory.
You'll need to download terminal from cydia.
Open terminal after it downloads.
and hit return
The password by default is: alpine
the password will not show while typing it, again hit return
Now type find . -size +100M
Make sure that M is capitalized
Notices the spaces find
CAPITALIZE THE M
This will now list anything with anything over 100MB. It will also list where the file can be found via ifile or winscp.
At this point I can't really help you out, its your choice on what to delete and what not to delete. If you really need some help you can post a screenshot here or In the original creators thread of this tutorial
and hopefully someone from imore or Modmyi can help you.
Now onto the last resorts.
Launch iTunes and remove all of your music, make sure you have an purchased music transferred to the computer, hopefully you don't have any music that is on your iphone that isn't on your computer.
Go into ifile.
Tap the "Gear" icon and tap "Hidden Files" to turn it on.
Tap "Done," then tap the "Back" navigation button to go all the way back to the root folder.
You should see a bunch of folders that begin with an "F."
Delete them all.
Hold "Home" and "Power" simultaneously to restart the device.
The other last resort
Var/Mobile/Cache I deleted this entire folder. It deletes all of your apps and all of your settings.