Resolution #1: I won't simply say "Be nice!" when my kid tries to hit or bite his friends.
Why it's worth making: Giving vague reminders when your toddler misbehaves rarely gets results. He's still learning what kinds of behavior are "nice" and "good."
Your new approach: Be specific -- and tell your child why your rules matter. Just make sure you keep the explanations short and simple; toddlers can typically understand something like, "We don't hit people because it makes them feel sad." When kids can see why certain limits are important, they're more likely to follow them. In some cases, you might try asking your child why he thinks a rule is necessary ("What do you think would happen if I let you leave your toys on the stairs?") before you tell him the answer. This will encourage him to start considering the results of his actions -- something little kids can't really do on their own.