Some of it depends on how much you eat spicy food.

Otherwise, I go by these rules:

• Eat vegetarian food for the first few days - or the whole time. It's so good in India, particularly the south, you don't find yourself missing meat.
• Dhal and curd rice are great for settling your stomach - and a good general accompaniment to make sure you have a get out clause should something prove hotter than you imagined.
• Be paranoid about water. Make sure you only drink water from safe sources - and make sure you see the seal on the bottle. If something has been washed in water, be wary. Watch how locals drink water - without the bottle touching their lips.
• Look out for how your cup/glass has been washed.
• Look out for mist sprays to cool eating areas down. You never know what the source of this water is.
• Verging on the paranoid again, but brushing your teeth using bottled water can help.
• I tend to trust Indian food a lot more than "Continental" food.

Following these rules, I haven't gone badly wrong. I'll make exception for early meat if I'm having a mutton biryani in Hyderabad (sorry, it's just hard to miss out, and I've never gone wrong with Paradise...)

Personally, I don't have a problem with street food (after acclimatising a little), but you need to follow your common sense, which you probably have from your intro. Is it popular? How hygienic is the preparation? How is it cooked? Are the plates/glasses disposable? If not, how are they washed? How clean are your hands? I know it's a risk, but I hate to miss out on vada pav, kathi roll, pani puri, all sorts of chaat and bhelpuri...