If you stay on top of how much is in your account, you're more likely to stay in the black
You'll usually get a monthly statement from your bank, either through the post or online. This will show the balance of your account on the day it was produced; all the payments in and out of your account; interest that's been paid or charged; and any bank fees.
Check the statement against your own records and if you think something is wrong tell the bank straight away. Tick off the transactions and work out if there are still payments or credits to go through. This will help you see what you have left to spend.
Or you could get a mini-statement from the cash machine at any time, but remember that the balance may not include anything you've spent in the last few days.
And remember, once you've checked everything shred any documents you don't need to keep - this helps protect you against identity fraud.