What happens when a reed is broken in?
Like many things about reeds, what happens when you 'break in' a new reed is surrounded by a good deal of myth, mystique and arcane mumbojumbo. But what happens is actually quite straightforward.
First, let's see what a reed is made of. Get a piece of gouged (not profiled) cane. Put one end in a glass of water and blow through the cane. You will see a vast number of bubbles coming out of the end. Thus, you can see that cane is made up of hundreds of tiny tubes along its length. When it's growing, the plant uses these tubes to transport water and nutrients to its leafs. These tubes are tiny - I'm guessing, but maybe a tenth of a millimeter or less in diameter, but they can and do absorb water.
Next, Arundo Donax (the plant that we use for cane) is a species of bamboo and related to crops like sugar cane, so it produces sugars when it is growing. After it has been harvested these sugars turn to starch. (The same thing that happens to sweet corn if left hanging around at the greengrocers). When you first start to use a new reed, your saliva will have an effect on it. Saliva contains an enzyme which is the first of many chemical attacks on the food we eat. This enzyme is called amylase and converts starch into sugars. (Try this: with a nice clear palette, eat some plain boiled potato and after chewing it for a few seconds see how sweet the taste it leaves in your mouth is after a couple of minutes. This taste is the sugars created by amylase converting the starch). This process happens to the starch in a new reed and the sugars are then dissolved away. So the cane will change a little after it has been used the first few times.
The other more important thing that happens is that those tiny tubes get clogged up by the dead skin and other stuff that comes off the lips and so the reed absorbs less water. This makes it harder (i.e. more resistant, less complient and less responsive). A brand new reed need only be soaked in water for a second or two to work but will, after as little as 10 to 15 minutes, become water-logged. So, never make adjustments to a new reed for longer than this. If you put a dab of nail varnish on the very end of the reed that goes on the crook, water will be absorbed less easily and the reed will blow in more quickly although, conversely, it will be worn out earlier as well. Another way of hastening the blowing in is to wipe the blades between finger and thumb so that some grease from the skin rubs off on to the ends of those tiny tubes thereby hastening the clogging-up process. Before they are fully blown in, some reeds are prone, in pianissimos, to making that fuzzy 'frying bacon' sound. The only thing that I know of to get rid of this is to apply a tiny amount of grease (lip-salve or cork grease or Vaseline petroleum jelly) to the inside of the reed for the first millimetre or so. Do this very carefully with a plaque. Although this is a useful 'quick-fix', I think that it makes the reed a bit bland and lifeless when it's in its prime and probably shortens its active life. In the same way, if you've used Chapstick or Lipsyl to stop your lips from drying out and chapping, always wipe it off your lips before playing or the longevity of your reed will be drastically compromised.

When is an old reed too old?
I consider a reed to be near the end of its useful life:
1) when the wires on the dry reed appear loose because the cane has shrunk back and it needs to be soaked for more than a minute or so for the wires to become tight once more. Or
2) when you need to keep re-wetting in water during a performance. Or
3) when tell-tale notes that tend to be sharp are causing too much trouble. Or
4) when the pitch of the crow of the reed has risen in pitch by a tone (if, say, it started its life off on an E flat and becomes an F).
At this point, the reed should be moved out of active service to join its fellow retired combatants in an old shoe box somewhere at the back of a wardrobe. Don't even think about trying to explain to your wife or husband, if you have one, why you can't bear to throw away an old reed; they will never be able to understand.
There are some methods to keep an old reed going, but I regard them all as less than satisfactory.
1) You can scrape the reed to loosen it up.
2) You can dip it in hydrogen peroxide (a bleach). This produces a dramatic fizzing but only limited life extension.
3) Some players use an ultra-sonic water bath. These are used by jewellers and opticians and maybe it does help a bit.

SuperGlue a Split Reed:
If you have a particularly promising reed and you have an accident with it and it becomes split at the end, it might be worth trying this little trick. Get the reed quite dry, then, using a cocktail stick or a splinter of cane from when you have used a shaper, put a tiny dab of Super Glue on the crack and then wiggle it around until it works its way into the join. Leave it in exactly the right position for a few minutes before trying it. It sometimes works and is certainly worth a try.