Eyeglasses are fundamentally rather fragile, seeing as they are made out of thin, carefully shaped glass and thin, malleable metal (or brittle plastic).
I have worn glasses for the past nine years, and I have accrued a great deal of experience in dealing with the many possible ways eyeglasses can fail.
In order to deal with breakage, the myopic and the farsighted usually learn to carry eyeglasses repair kits. A good one will contain a tiny flathead screwdriver as well as several different sizes of itsy-bitsy screws and nose pads replacements.
But that's not all you need to really fix your glasses. There are times when one will need paperclips, adhesive tape (scotch, masking, duct or otherwise), and an oxy-acetylene torch.
An imagination, and as many episodes of MacGuyver as possible would also help.
Some examples of ingenious fixes to breakages I have come up with follow:
When a screw from the lens holder in the frames is lost, and your precious ground glass tumbles to the floor, a paperclip inserted into the hole and bent properly will hold the two sections of frame together nicely. In fact, it proved to work so well that I never replaced the screw (it's been almost a year).
The elbow joint hole threads sometimes strip. In these cases, I have tradtionally used tape. cjeris recommends using nylon cable ties, as they stay tight for longer than tape and survive flexing better than cyanoacralate (crazy glue).
Sometimes that elbow joint will just snap. Re-screwing it is impossible, of course. This is when you will use either the tape or the oxy-actelyene torch. With glasses, tape always has to wrapped very tightly, lest things jiggle free.
One time, one of my little sisters kneed the other in the face and broke her glasses at the nose bridge. It was awful. We used tape...she may have looked like a doofus, but at least she could look!
Avoiding eyeglass breakage can some effort take, but it is almost always worth it. If you are going to ride a rollercoaster, remove your glasses and place them in the (usually provided) cubby hole. If you are about to be punched in the back of the head, remove your glasses (actually, mine didn't fall all the way off my face, and I was able to snatch them while I was fleeing). If you think you are going to head a football, quickly slip them off. Never wear your glasses while sleeping. Bend your glasses so they grip your head tightly if you are going to mosh.
Always take care, and I remove my glasses for emphasis when I say that.