Clothing:
Wear loose-fitting clothing that will not rub against the Tattoo -- especially in the first 2 or 3 days of healing.
It is better to keep your freshly tattooed skin exposed to the air during the healing process if at all possible, and if not, to avoid tight clothing that can "stick" to the Tattoo, or clothing that will cause you to perspire where you have been tattooed, etc.
Tight Clothing: Again, a little bit of extra care in the first 48-72 hours until a good "skin barrier" has formed will promote faster, cleaner healing. It's only common sense that wearing tight fitting or restrictive clothing that rubs or irritates the freshly tattooed skin is going to lead to scabbing. Additionally, bacteria and other foreign materials embedded in clothing can become a further source of infection for freshly tattooed skin.
THINK AHEAD -- If you are going to your thigh, lower back or buttocks worked on, don't just think you'll wear your jeans home -- bring along a pair of sweatpants or track pants that are loose and breathe that you can wear instead.