The sales slip must include both a cardholder signature and the card account number to be valid. The account number must be obtained directly from an imprint of the card itself or from electronically reading the magnetic stripe. Manually entering the account number does not protect you from a no-imprint chargeback even if the sales slip is signed.
Quick tip: by programming your terminal to request the last 4 digits of the card, it quickly checks to make sure the card swipe and the magnetic strip are the same. If you override this function, you may be more vulnerable to losses.