How to make a huge rainwater cistern

We had less than 1" of rain between October 2012 and July 2013.  But, now that July is here, the rain season has started, which means that it is Spring, here!

We received about 2.5" of rain in the last 48 hours.  The rainwater cisterns are filling up!  We caught 4,000 gallons during this period, which is enough to keep our garden watered for an entire year.

Here's our roof potential:
House Roof + tank roofs - ~1,000 gallons of water per inch of rain.
Barn + barn tank - 500 gallons of water per inch of rain.
Sheds and assorted tanks - 150 gallons per inch of rain.

TOTAL - 1650 gallons per inch of rain

For capacity, we have the following:
House cisterns - 14,500 gallons + new tank(7,000 gallons, under construction)
Barn Tank - 7,000 gallons
Shed tanks - 1,500 gallons
Swimming pool - 2,000 gallons

TOTAL - 32,000 gallons

So, we need about 20 inches of rain to fill everything, if they start empty.

BUT, our water accounts are about:
House Tanks - 6,000 gallons
Swimming pool - 1,000 gallons
Barn tank - 1,500 gallons
Shed tanks - 1,000 gallons

TOTAL - 9,500 gallons

So, our current capacity is 32,000 - 9,500 gallons - 22,500 gallons, which translates to about 14 inches of rain.

We typically receive about 25 inches per year, and I think we're now around 5 inches for this year (4 of that is this month).  We can reasonably expect 20 inches of rain between now and the end of October.

Rainwater catchment information