How can I concentrate citrus juice at home? I used to make no-bake pies as a kid that consisted of whipped topping, condensed sweetened milk, and frozen lemon- or lime-aid concentrate (thawed) mixed together and poured into a graham cracker pie crust then chilled to set.  

The recipe I learned called for a small can of frozen concentrate, a can of sweetened condensed milk, a 12-16 ounce container of whipped topping and a 9 inch pie crust.  But lately all I can find are large cans that result in a runny pie filling that doesnt set well.  

I would like to try with homemade lime or lemon concentrate but how would I go about concentrating the juice?
Some fruit juices can be heated to drive off excess water (especially those with a higher sugar content) but citrus doesn't tolerate this well.  Fresh-squeezed citrus will get bitter and acrid if reduced.

Instead, squeeze your citrus as normal and freeze the juice in an open container.  Once it's set into a solid block, place it into a funnel or strainer over a collection vessel.  The juice itself will run off more quickly and collect in the vessel, while the ice crystals stay in the strainer (until they melt, at least - don't let them melt all the way, or you're back to the beginning).  Once the ice looks very lightly colored, discard it and enjoy your concentrated juice.  You can repeat this process if you want further concentration, though eventually there will come a point where the juice is so concentrated that it won't freeze at conventional freezer temperatures.

Good instructions for this process here - it's really quite simple!