As others had said mate dressto what you would normally, Personally i couldnt go to a club in a tshirt, shorts and sandals but depends how you roll i suppose! Has to be Smart for me, Even in the UK you make an effort, but your there to party!! Im going to take white leather shoes/plimsole type, Havianas in white or black could be a pass, but the broken glass is an issue!
What about ladies? I don't want my nice heels getting messed up, I thought about sandals until the glass comment. Now I'm thinking boots or wedges?
As the others mentioned, anything goes...I'm leaving the expensive shoes and jewelry at home though...no need to bring it and it also catches the attention of locals (never know).
Lets not be so closed minded. Expensive material possessions catch the attention of EVERYONE, not just the locals. Believe it or not, not all of the locals are as evil as the news or reports would have you believe.
Sorry to digress but following on from fletch's comments i just want to add that i was reading an article on mexico and it showed the stats and geographical locations of the murders. Most of them happen in the places closest to the mexican US border. It's funny my friend was asking me if he sbould bring his piece of jewellery. I said yes he should if he wants to. I have never felt threatened in cancun.