Making your clothing selection depends largely on what you plan on doing. If you are staying in Cancun or on the Riviera at a luxury resort then expect everybody to be dressed nice. Vacationing Europeans are extremely fashion conscious, even on the beach. You need to coordinate an eclectic wardrobe to be successful here. You can also buy some new items while there.
If you plan on hanging around Playa and laying on the beach all day nobody will bat an eye as you walk into a restaurant in shorts, tank top and scruffy hair. In other words you can tone down your wardrobe considerably and walk around in your pajamas all day if you wish.
For backpackers less is best, wash as you go.
RECOMMENDATIONS & TIPS
Take beach shoes with you for walking in the water and swimming. It is a common occurrence for vacationers to step on stingray tails, sea urchins and other things.
Short sleeved shirts are better to take than t-shirts. On extremely hot days you can unbutton your shirt and allow better cooling. A shirt also doubles as dressware for restaurants.
Many people recommend 100% cotton however we have found that 100% Viscose is the best material for travel clothing for hot climates. This material is super light, does not crease and lets you breath so you do not sweat. Cotton is recommended for underwear and socks.
Zipper lube. Humid weather will in time reap havoc with your suitcase/backpack zippers. Protect your zippers by coating them with zipper lube, which can be purchased at most dive stores. A candle will do the same trick in a pinch!
Buy an Under Water Camera if you plan on swimming or snorkeling. You will be amazed at what can be found 20 feet from shore on some beaches.
If you have a printer print off some personal cards with your name and e-mail address for staying in contact with friends you make.