Try cutting a business card the size to fit on top of the battery, then put the door on slide it up and it will be much tighter. (BB Storm trick) Still works though, I needed something since I dont want a case and I pocket my phone... Hope this helps.
AD
i used the business card trick with my blackberry (and post-it note, and receipt paper, and $1 bill, and... etc..) battery cover. what ended up working for me was electrical tape, i guess because it is tacky. put a piece of electrical tape dwn the entire length of the battery, and see if that holds. if not, chunk that piece and put a SMALLER PIECE UNDER A LARGER PIECE to create a "hump" of sorts on top of the battery. you shouldn't have to layer more than two pieces, but that should work like a charm.
disclaimer: my battery cover doesn't have this problem, so i have not tried this as a solution ON THIS PHONE, but it has worked for me on another phone in the past.