The following items are recommended for your stay in India and/or Sri Lanka.

10 sets of loose, comfortable clothing (e.g. kurtha tops and loose pants, punjabis)
1 set of warm clothing (e.g. jeans, vest, long-sleeved top and jersey)
1 warm jacket
1 raincoat
5 pairs of socks
1 pair of comfortable, slip-on walking sandals
3-4 sets pyjamas (at least 1 warm)
2 towels
sun hat
money belt
wall socket adaptor (for hairdryer, camera charger, etc)
toiletries (include soap, shampoo, facecloth/sponge, toothbrush and toothpaste, etc)
insect repellent (essential)
2 rolls toilet paper (toilets outside the hotels seldom have loo paper)
personal journal
camera, laptop, etc.
reading material for long periods in the bus
plastic bags for laundry

Important: First Aid Kit
Each traveler should have their own first aid kit.This must include plasters, bandage, Immodium (for diarrhea), pain-killers, antiseptic ointment, rehidrat, glucose packs, antacid tablets, an antihistamine (for allergies). Any prescription medication which you are taking (remember to include prescription).