Luggage and bags:
Bring with you extra luggage space. You often end up with having to buy an extra bag
Clothing/Shoes/Weather Gear: As for any trip in the Med Sea
Toiletries and Medical Supplies: extra antibac and some hygienic stuff can be ok. But there are ok toilettes around also.
Miscellaneous: As you're on VT probably you prefer to travel alone with no organized trips. And probably you're already an experts. But for the ones that are not I wanna say that Istanbul is very easy to be tortuous by yourself. Transportation is easy and the landmarks are close to each other. You just need a travel book. And if you don't have the chance or you forget to buy it before your travel don't worry - you could have one there.
One of the common questions from women is about dressing respectfully as many of the places people visit are mosques. I saw women wearing everything from shorts and a tank top to the local Muslim women who were wearing coats and head scarves. All of the mosques have scarves and coverings should you not be dressed appropriately so I'd advise just to wear what you are comfortable in. I brought light colored lightweight tops and convertible pants (the kind you can roll up to make capris) plus a lightweight skirt, I was warm but I think I would have been warm regardless.
Bring a hat, the one day I didn't wear it the top of my head burned.
Comfortable walking shoes are a must
Miscellaneous: The basic but sufficient inner city Istanbul map is available from the tourist office near blue mosque. Chcek out if it is available in the langauge you speak.
Luggage and bags:
A good page to start preparing your trip is Istanbul.com
Here you can find loads of info about weather, monuments, restaurants, maps, links, hotels...
Clothing/Shoes/Weather Gear: Be prepared for sudden changes in the weather. Carry an umbrella with you in case of sudden showers!
Miscellaneous: Bring a thick skin to Istanbul, which is neccessary when dealing with aggressive Turkish carpet dealers and other misc. con artists on the streets of Sultanhamet.
Toiletries and Medical Supplies:
Toilet paper. A few sheets with you, all toilets do not supply.
Miscellaneous: Suggest lonely planet Istanbul encounter UK £6.99. USA $11.99, as a good compact guide book with map.