Tipping 

Visitors are not to be expected to tip taxi drivers. However, hotel, airport and train station porters should be tipped approximately Rs20/bag.  If a service charge is not included, tip guides Rs 500 and drivers approximately Rs300 per day or 10 percent where appropriate. In restaurants, if the service was good, tip anything between approx 5-10 % of the bill. Depending on the category of the hotel, and the bill amount. Its not mandatory, you may not leave a tip if you don't want to. 

Etiquette

Delhi is not a highly conservative city. But while traveling in certain places like Old Delhi, it is better to dress conservatively. One should not try to attract a lot of attention towards himself/herself due to his/her dressing style. When entering temples, mosques or buildings of religious importance always check what dress attire is required.  In most cases you will be required to cover your head, take off your shoes and be dressed conservatively. Its always a good idea to enquire first at your hotel or with your guide about the dress codes (if any) of the places you will be visiting before you set out. Please keep your mobiles in silent mode when you are visiting such places. Also public display of affection is not recommended. Hugging and kissing in public places is a strict no no.

People understand and give a certain leeway to tourists, but it doesn't harm to respect local customs. 

Be respectful towards seniors.