Carrying around a checkbook to keep track of all your debit card purchases is a pain, and thus something likely to be skipped, leading to inaccurate balances.
Print a business card with all the numbers from $500 to $5 counting down by 5's. Then you round your balance DOWN to the nearest 5, and cross out all numbers greater than that. Then when you buy something you round UP to the nearest 5 and cross out that many more numbers on the card. This gives you a balance estimate that will always be at least a little bit more than you really have in the account, you can get a new card whenever you deposit, or balance your checkbook. On the back are several lines to quickly note the actual price if you wish.
I have these coming in the mail, figured it might be worth a try so I made up a test batch, and I'm expecting them in the mail in a week or two. If anyone wants a couple of them to try as well, let me know, I'm getting 250 of these.