One of the harder things to do for all-grain brewers is brewing high gravity beers.  These beers push the limits of our equipment and also raise the possibility of collapsing the grain bed due to the large amount of malt.  Even with a successful mash, the brewer is faced with collecting only higher gravity runnings leaving a lot of good sugars still left in the mash (you could continue to collect but your starting gravity would suffer).  A person with an efficiency of 70-80% can see their mash efficiency drop to 50% for high gravity beers.

Why let all of those good sugars go to waste?  By simply doing what brewers used to do long ago, you can get a great second beer out of those grains with just a little effort.  When sparging the first "strong batch," maintain a water level on top of the grains all the way through the point when you have collected the desired amount for your strong beer.  This means you will have to have more sparge water available to start (assuming that you typically drain through the bed at the end of a normal sparge).  Take your first "strong running" and get that brewing. 

If you don't have another large pot, you can just let your mash sit for a while.  If you have another collection vessel, you can add a second batch of sparge water into to your mash and start collecting the second runnings.  This batch will be significantly weaker than your first batch (you should monitor the sugar and pH of your runnings to make sure you aren't getting too weak or raising your pH to a level which would extract tannins), but still be good.  I usually add a small amount of citric acid to my sparge water to prevent the pH from getting too high during sparging. 

After collecting everything, check the overall specific gravity and determine how much of the wort you need to boil off.  (Example: you have 6 gallons of runnings at 1.030: to get a beer of 1.040 you will need to boil down to 4.5 gallons, 6 * 30 / 40 = 4.5)  Alternatively, you could add some extract to get the strength up.  I have also added some extra grains after my first sparging (in this case I had to wait for them to go through conversion as normal before doing my second sparge).

After all is said and done, the second runnings may end up being a little astringent or grainy, but for an extra several gallons of beer, you can look past that.  With the second runnings, try to aim for a nice session beer (English bitters usually work well).  I've brewed an Imperial Stout and made a second runnings porter.  If you think it may be too light in malt flavor, try shooting for a more pronounced hop profile, aiming in particular for your flavor hops.  Using this technique, your extraction efficiency will jump back up to where you normally have it and you'll have something nice to drink soon while you wait for the stronger beer to age.