If your space is mainly being taken up by apps, there's little you can do, other than clearing out the app's files/data/cache (jailbreaking makes this simple). If you're storing a lot of personal files or media onto it, an external storage device would work. Some use ad-hoc Wi-Fi for connectivity and microSD cards (or HDDs/SSDs/non-removable storage) for actually storing things.
If jailbroken, you can directly access the filesystem of an SD card when using the SD card adapter, which is fine for media and personal files, but can't be used for storing apps onto with symlinks and such.