u can try install some boss pref toggles from cydia or manually to disable some of the services such as ilog, winterboard, pdanet and bla bla bla. these services are just fine to disable it in order to avoid mass memory consumption. Another option is install sbsetting (which is more convinient to launch if compare to boss pref) and always check ur available memory there. U may resprint ur iphone if u notice that ur memory runs under 30Mb. however, sbsetting might not have the some of the advance function as boss pref did.