USB Flash Drives formatting

I have two usb flash drives (1- 256MB Lexar JumpDrive, 1- 64MB drive from AVB). What is happening is that whenever I save a file to either of them, and then remove the file later, the amount of space available stays the same, as if I'd never removed anything. So whenever I try to save something else to the drive, I get a message saying that I don't have enough space on the drive.

I've tried formatting them and get the same results. What would be the proper format for such drives and Mac OS X (Panther)? Why is this happening?

It's probably not a formatting issue, it's probably that your flash drive is storing a backup or a trashed version of the file on the flash drives, even if you trashed the file. Try exposing hidden files (I admit that for one particularly pesky flash drive, I had to plug it into a PC to see them.)