Packing for Istanbul

Clothing/Shoes/Weather Gear: Even in summer, expect to wear long pants (or long skirts for women) and shirts with at least short sleeves. This is because mosques forbid shorts and short skirts and bare shoulders and you will likely be touring mosques during your stay in Istanbul. Also, most Turkish men do not wear shorts, so you will stick out much more as a tourist if you are wearing shorts. Also, women should bring a scarf or two to carry in your purse so that you will have one to wear when touring mosques.
Toiletries and Medical Supplies: Bring plenty of toilet paper, since public bathrooms often do not have (or run out of) toilet paper. Also bring wipes and hand cleaner. If you have fair skin, also bring sunscreen, since you will do a lot of walking outdoors in Istanbul.

In terms of medical supplies, bring something for diarrhea and food poisoning. We brought Immodium and did need it once in our 10-day visit.
Photo Equipment: Bring lots of memory card capacity, extra batteries, and a charger. You will probably take a ton of photos and videos in Istanbul, and will burn through memory and batteries more quickly than usual. Photo equipment is available for sale in Istanbul, but is much more expensive than in the USA.
Miscellaneous: Bring the following guide books:

The Istanbul book from the DK Eyewitness travel series. It has lots of photos and info about not only the main tourist sights, but also about numerous less well-known sights thrughout the city.

Rick Steve's Istanbul book. It contains well thought out self-guided tours through some of the main tourist sites.

Top 10 Istanbul. This book has numerous top 10 lists (Top 10 mosques, Top 10 museums, etc.) that help you make decisions about how to spend your time.