Greet people with a warm Namaste (or "Namaskar" to an older or high-status person) with palms together, fingers up. Show marked respect to elders. Be friendly, be patient.
Feet are considered dirty. Don't point the bottoms of your feet (or your bum!) at people, or at religious icons. In this vein, be sure not to step over a person who may be seated or lying on the ground. Be sensitive to when it is proper to remove your hat or shoes.
The left hand is considered unclean because it is used to wash after defecating. Nepalis carry a small jug (called a lota) of water for this purpose in lieu of toilet paper. It would be insulting to touch anyone with this hand.
Circumambulate temples, chortens, stupas, mani walls, etc. clockwise. (ie, with your right side closest to the object or respect)
When haggling over prices, smile, laugh, and be friendly. Be prepared to allow a reasonable profit. Don't insult fine craftsmanship, it's much better to lament that you are too poor to afford such princely quality.
Many Hindu temples do not allow non-Hindus inside certain parts of the temple complex. Be aware and respectful of this fact, as these are places of worship, not tourist attractions.
Being a non-Hindu makes you moderately low caste, but not an untouchable. Avoid touching containers of water; let someone pour it into your drinking container. Likewise avoid touching food that others will be eating. Make sure you are invited before entering someone's house. You may only be welcome on the outer porch, or in the yard.
Wash hands before and after eating. Touch food only with the right hand. The left hand can be used to hold glasses, bowls, and probably eating utensils. Outside the main cities, be prepared to eat rice meals with your (right) hand as Nepalis do, or bring along a fork and spoon.