Yesterday I did a battery pull and had 10782350 bytes and at the end of the day with no applications running (I used alt+escape) I had 2589465 bytes left.
do you close programs by hitting menu>close? that is the only way to completely close a program, otherwise it will stay open and eat up your free memory.
have you recently cleaned your browser cache? if you use your browser everyday, it also eats up alot of memory.
What OS version is on your device? In Options --- Status. If it's 4.3 I would recommend upgrading to 4.5 (sticky at the top of this subforum), it manages memory much better. Grab Crackmem (do a search here for it) as well and use that to free up some junk.
Also, do you have facebook on there? That's been known to eat memory like nobody's business.