Remember you are in a Muslim country, so dress conservatively if you visit any mosques etc. Don't get drunk. Drink bottled water, and avoid salads, as they are washed in tap water; you may be ok, but I had a bad experience on one of my visits to Istanbul. This is one city where the tourist bits live up to the expectations, and the food (minus the salads!!!!) is good. One thing I'd recommend: go to a Turkish bath for the full treatment; you'll feel like you're getting beaten up, but afterwards I felt great!!!!!! When shopping in the souks, remember to bargain with the shop keepers, they expect it, and will think you're stupid if you don't try to get a good price.
the only tip is go! And walk, watch, eat, speak... Istanbul is one of the most fascinating city in the world, Turkish are kind and hospitable.