Miscellaneous: Forget your wife, your husband, your girlfriend, your boyfriend, your mother, your father, your sister, your brother or any other travelling companion. These little things will be your best friend throughout your whole time in India. Look after them, nurture them, feed them , water them and by all means don't lose them!
Clothing/Shoes/Weather Gear: Light cotton wear in Summers (April to Sept) and Woolens in Winters (Nov-March)
Toiletries and Medical Supplies: Mosquito Repellent (specially in Monsoons - July and August) and keep a stock of toilet paper as you may not find that in some places (actually you'll have to work hard to find it)
Miscellaneous: Torch and an Army Knife