well as we know htc explorer provides too less internal space regarding this i have some suggestions!
1) use some app to move the applications to move to your sd card do that every time if you use too many apps ( i use app 2 sd download it for free from android market )
2) try to clean up cache every time.as all caches are mostly stored in to device memory (you can do it from settings > apps then select individual apps and clear cache )
you can use additional apps for this available in android market for free! i use ZDbox
3) disable apps that you don't use that one consumes power also (use apps like advance task killer,ZD box all in one,task killer)
4) delete the browser history and cache also reduce web cache capacity to minimum (default 6MB)
5) don't flood your phone with too many apps that may slow down your device and also most functions.
6)disable the auto sync functionality from settings > accounts & sync that will consume lot of memory as per connected accounts like HTC sense,facebook,twitter,gmail,mail box etc.
7) delete matched contacts which stores additional informations in your contact list.delete un used contacts and email links!
there are many utilities you can solve that memory problems but it needs some compromises!
well hope you got that all !! have fun. email me if you have additional problems i will try my best.