How can I repair a slow drip at the main water shutoff valve?
Have you tried gently tightening the gland nut? It's the hex cap under the handle and squeezes some sort of packing against the valve stem. Tighten only in 1/16 turn increments and only if it moves with minimal force. If it acts in any way like it's tight, somebody else may have already compressed the packing to its maximum and no further seal is possible. 

This is a globe valve and its design dates from the days of steam for a good reason. Globe valves usually have arrows on them to show proper installation, the inlet should always be the pressure side with the valve closed.

If properly installed so the pressure comes in on the bottom of the valve, with the valve fully closed, the stem is on the pressureless side and allows you to remove the handle and the gland nut to replace the packing without taking the system fully out of service.

Packing is available at any plumbing store, take the gland nut and what you find under it with you so they can identify what was used.