Balance your checking account monthly and update your register after each expenditure.
Sign up for overdraft protection on your checking account.
Beware of debit cards when your balance becomes lower. Debit cards generate "holds" on your account which can trigger overdrafts.
When your account balance is low, consider making purchases with a credit card rather than with a debit card or check. Then, pay the credit card off.
If you determine you have overdrawn your account, immediately transfer funds from savings or make a deposit to the account. Do not use the account until any transfers or deposits are considered "good funds".