I wore black slacks (not the super tight variety you see in Madrid) and a loose fitting, plain (no logos) short sleeved t-shirt. You could also wear a longish skirt. (We got to ride camels, and that's not such an easy thing to do in a skirt
) I don't recommend short, short skirts, SHORTS, or any of those spaghetti strap, halter, or otherwise body-baring tops. I wore sandals, though some members of our group wanted their feet protected against whatever they might step in (?!?!?!?) You are going to stand out as a foreign woman anyway, but I think it is wise to be more conservative in one's dress out of respect for their culture.
Hope this helps and have a great time!