Clothing and shoes
Avoid wearing revealing clothes (especially in rural areas). When visiting a religious building, you will generally be required to cover your head and be dressed conservatively. In addition, some Hindu temples may not allow any leather items inside the temple.
Make sure to remove your footwear before entering a temple/mosque. Traditionally, shoes are not worn in households either.
Hand and feet
Only use your right hand when receiving/eating food or when receiving/giving money or gifts.
Avoid touching books/papers (money/gold) with your feet. They are considered the physical forms of the Goddess of learning (Goddess of wealth).
Avoid touching or pointing at people with your feet. Feet should also not be extended toward any religious artifact or symbol.
Avoid displaying affection in public. It can lead to fines (e.g. Rs 500 in Delhi for making "illegal use" of public spaces) or even arrest.