If you aren't the type to write down every debit card purchase at the merchant's counter, keep your receipts in one place. Put them between the pages in your check register or in a designated envelope, and then update your checkbook every few days. Or, start keeping track of your transactions on the back of an ATM receipt and wrap it around your debit card. Every time you reach for your card, you will be reminded to jot down the amount. Designate an overseer of your joint account.
Somebody needs to do it.
If you share an account, make sure one of you has the responsibility of keeping up with it. Try to agree on where you will keep receipts and what checks will be written each month. Use cash for the little stuff. Get checks with carbon copies. A good way to keep track if you write a lot of checks.