Maintain your balance, especially after being hit. If you are off-balance, you are venerable to more attacks, or falling, which may injure you further and leave you even more vulnerable. Keep a stable stance and use minute body movement to maintain your balance. Large movements can leave you vulnerable and even cause you more instability.
Do not let the first punch you take be one that was thrown in anger by an attacker. If it is, it may be the last punch you ever take.