Casual is the key. Miami is incredibly laid back and most places allow shorts, tanks, bare shoulders/arms and even flip-flops.
People wishing to "Be Seen" on Lincoln Road may wish to dress up some. Seems to be the standard for clubbing. Follow general rules you would at any large city as NYC or LA but keep in mind that Miami's Hot tropical nights are S T E A M Y ! ! ! And I don't just mean sexy!
Plenty of stores and boutiques selling latest fashion trends and bathing suits
Photo Equipment: Digital is the way to go, remember to carry a good UV and Polarizer if you have an auto focus 35 mm lens or comparable.
Camping/Beach/Outdoor Gear: Suntan Lotion with Aloe additive. A good high rish factor sun block to protect you against the harmful UV rays of the sun.
Miscellaneous: Sun Glasses and a money belt! Don't walk through crowded south beach or go to clubs w/out a money belt in your pants.
Lastly, remember to photocopy all your important papers in triplicate; One for your wallet, one inside your suitcase, one in a purse or carry one and originals in your hotel room safe.
Include your passport, driver's license, traveler cheques numbers and important contact info.
Luggage and bags:
SOME OTHER GOOD PAGES ON MIAMI