Rear hub repeatedly coming loose My rear hub keeps coming loose which causes the rear wheel to wobble.

I'm pretty sure that I know how to tighten the cones properly and I've also had the bike serviced in my local shop. Every time the wheel will be fine for about 100km and then it will start to wobble again.

What could have happened to cause this, and how can I fix it? Could replacing the axle, cone nuts and bearings help?
Either your cones are coming loose, or the bearings are absolutely fried.

I had this same thing happen to me when I was first getting into bikes.

The hub was loose, I tightened it, I went for a ride, the hub was loose again.

I finally got up the nerve to take the hub apart (yeah, I was pretty n00b back then). The cones and cups were damaged unbelievably. And a stray bearing was well on its way to eating through the axle.

You've had the bike "serviced"... did they take the hub apart?

If the cones are coming loose, you need to lock them in tighter and make sure that you're adjusting correctly (just loose enough that there's no binding. A little play is okay).

In the case that the cups and cones are completely fried (at the rate that the wheel is coming loose, they would be completely fried), you need a new hub.