Don't drink the water!!! All hotel rooms have plenty of bottled water so make sure you use that to clean your teeth. I was in a 5 star hotel, but the first time I turned the cold tap on what came out had a brown tinge and was slightly thicker than I was expecting! Make sure you have plenty of Imodium: I cleaned out my local Asda.
Don't have ice in cold drinks, because it's often made using frozen tap water. The same with salad. When you go out, make sure you put some toilet roll in your pocket in case you get caught short, because not all toilets have it in, especially outside the main parts of the cities. I had business in the south and would definitely have wanted to know that before I went!
The traffic is incredible and there is no highway code as we know it, especially outside of Delhi. I saw a motorcycle pulled over with four people on, and the reason they stopped them was that they weren't all wearing crash helmets! They were ok with the number of people. Outside Delhi they wouldn't have stopped them at all. The chauffeur picking you up will earn his money. The car will go through spaces you don't think are big enough for a bike!
See how many people you can see on a tuk-tuk. The most I saw was 15! ( A tuk-tuk is a tiny Piaggio truck used for carrying people round. They cram incredible numbers of people in them) It's normal to see 12 people in one. I saw 14 on one, but two of these were banging drums and everyone, including the driver, was dancing!
When you are flying over, if you have a window seat try to be awake when you go over the Himalayas. The view is stunning, especially if like me you have the computer screen showing how high you are. We were only over the foothills but they still weren't that far below us at 30,000 feet.
This year isn't as hot as last year - it's struggling to reach 45C at the moment! Definitely don't forget the factor 50.
I don't know who you're going with, so don't take this the wrong way, but public shows of affection are definitely frowned on! I was lucky enough to go to the Taj Mahal, and we stopped at services on the motorway. A couple of tourists were kissing beside their car and my driver was worried that locals would see them and kill them (literally - they lynch people in some areas for this)
Make sure you have extra memory cards for your camera. Enjoy the experience. It is incredible, especially if you have not been anywhere like it before - I found Istanbul amazing, but India is a whole new level!
Don't drink the water!!!!