How can I concentrate citrus juice at home?
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!