There's a number of recommendations to make - some are common sense, some verging on the paranoid, but really, it's down to the individual and the state of their immune system (i.e. someone who spends their life spraying antibacterial spray on everything will likely fare worse than someone with a bit more balance).
However, what I would suggest, based on my own experience (I go out there every few months):
- The general principle is that, even if something isn't dirty, the bacteria present are not the ones that your body is used to, so you can't just go by eye alone, at least not for the first week.
- Be very aware of water. If you're buying bottled water, check it's sealed, and try to master the art (as you'll see locally) of drinking from a bottle with a slight gap between it and your lips.
- Be aware of indirect sources of water. Ice is a classic - but also think of what things could be washed in - whether the food itself (raw salad ingredients) or the vessel. Also look for water sprays - sometimes used to cool outdoor diners.
- Vegetables are generally a safer bet than meat (although a lot of meat will be fresher than at home). And there is a wealth of great vegetarian food available in India - don't overlook it just because you're used to having meat twice a day at home. I normally do this for the first couple of days of every trip (although it's often so good that I keep going...)
- People have different opinions and tolerance to spice - and of course there are different kinds of 'heat' (chilli, mustard, pepper, etc). Some people will tell you to lay off spicy food altogether - I'd say only do this if you are not at all used to it at home. There's nothing sadder than a dish that's supposed to be spicy with all the spice removed. That said, you may want to give the Andhra Country Fish a wide berth for the first few days. And you may wish to build up your tolerance (I do this, but mostly because my colleagues out there know how much I enjoy spicy food and have been know to test me...)
- Be aware of your personal hygiene - wash your hands well, always with soap, and don't bite your nails or put objects near your mouth, even if you have subconscious habits like this. Bear in mind that you may only get a spoon with some meals, and bread instead of rice (particularly in the north) so you'll be using your hands at some point.
- I don't like antibacterial products for everyday use. However, a small bottle of gel is useful for those situations where you can't get to wash your hands somewhere sanitary.
- Beware of buffets. You don't have to avoid them completely (they're insanely popular) but be aware how often dishes are being replenished and how hot they are. Lukewarm food sitting open for hours is not a good thing.
- If you do want to avoid spicy food, there is a lot of less spicy Chinese-style food available, as well as 'continental' food (i.e. European, usually banded together, and heavy on the Italian). These are often presented in 'multicuisine' restaurants, too.
- If you do get a sense of security later on in the trip, and fancy some street food or drink, do be careful. Look for how the food is covered and handled, how popular the place is, and - this is my pet obsession - how the serving utensils are cleaned. Disposable is usually safest, and metal can be very well cleaned, but the water may not be the cleanest. I once picked up a giardia infection (which lasted 6 months plus) due to a glass which I later found out had been rinsed in dirty bay water...
- Don't be paranoid. And don't miss out on some quite wonderful food - one of the best things about any trip to India. It's not uncommon to suffer a slightly upset stomach for a few hours - and it's not the end of the world if you do. Take imodium (or any other brand of Loperamide) to have readily at hand - but any common medicines can be easily found in pharmacies (and mostly given out a lot more freely than in the UK...)