If you plan on driving, remember that the Tow Away signs are serious and you will be towed. Parking is a very serious business in Miami Beach so be careful where you park and never park in alley ways. If you stay on the beach, utilize the public transit system. Taxi, shuttles or buses are very reasonable and you can always use your rental car to visit attractions outside of the city.
Never leave your items unattended. If you have a group, make sure someone stays behind to watch your valuable items. A little note, the loungers on the beach cost money. Park yourself there and expect to have someone approach you for payment.
Drink lots of water on those hot days and if you feel like having a beer or two, make sure you read the posted rules about alcohol. Haulover is pretty lax about the rule if you are discrete. Also watch your drinks. It's a good rule to use everywhere, but you never know when someone is trying to slip you something.
Pick up the guide book to find the weekend events and check the New Times (look for the red street side boxes)
Most of the restaurants add 15-20% directly on the bill especially during peak season like Spring Break. Don't be surprised if you see an extra fee tacked on your bill.