If a blister is not causing any pain or keeping you from walking normal, then just leave it alone. If it is causing pain, then you need to drain it. Remember that if you suffer from diabetes or poor circulation, consult your doctor before you do any self-care. The proper way to drain a blister is: wash your hands and the blister with warm soapy water, swab the blister with rubbing alcohol, sterilize a sharp needle, puncture the blister in several spots along the edge (not on top), let the fluid drain and keep the top skin intact, then apply some antibiotic ointment and cover with a bandage.
Never pop a blister or tear the top skin off. That top skin naturally protects the area from the blister getting infected.
Photo | Post marathon race blisters. Warning graphic content | © Ted Murphy | Used under a Creative Commons Attribution License