In Australia the majority of homes without mains water use rainwater harvesting to provide for household water needs.
Basically a large tank, anywhere from 5 to 25 thousand litres on average, but tanks come smaller, and larger, stores water collected from roof surfaces during rainfall.
The roof of the house, and any outbuildings such as sheds, garages etc etc provide an area for collecting the water, and its diverted using guttering around the eaves, and pipes, into the tank.
Unless terrain is perfect, (ie you have a roof, and the tank at a higher elevation than the plumbing in the house) you will probably need a pressure pump of some sort maintain water pressure inside the house. the best ones are usually impeller pumps with a bladder and pressure switch to maintain a minimum pressure. Theres nothing worse than a shower that dribbles, and only dribbles. (If you are relying on your own water showering should be short and the pressure as low as you can stand most of the time, but sometimes its good to get hammered by hot water, esp if you have been working hard and are covered in filth.)
The best guide I have found on this is here:
http://www.ircsa.org/factsheets/Cunliffe.pdf
It covers everything fairly well.
Far better than I could have.
Basically you want to keep the water of good quality. Especially if drinking it. The best filter you can find is a good idea, although I don't have one. But after 10 years using the same water supply I guess my body has adjusted to whatever minor organism/bacteria/other minor pathogens might be in our supply. I am not recommending being that slack to anyone else tho.
The main sources of contamination are obviously the roof, and guttering. If the tank is sealed to the outside save inlet and outlet ipes then nothing should get in there. If your tank is a below ground tank then make sure it keeps its integrity as soil borne contaminants are a much gretaer threat.
Roofing material has to be kept free of toxic substances, ie the material itself (no lead paint for example), airbourne contaminants such as soot/smoke from an indoor fire, from persticide and herbicide spraying in the area, general air pollution etc etc. The link above covers all this in much greater detail with more clarity. Animal waste, ie bird crap, etc etc is another potential hazard. Thats why post tank filtering is probably your best bet.
These days the majority of our roof catchment is under a massive avocade tree so we keep our rainwater disconnected. We fill our tank from a bore/spring set up and that water is pretty good as well. We disconnected the rainwater after someone who stayed with us got Guiardia (sp) a water bourne nasty that is primarily transmitted through bird crap. We never suffered from it tho, but its not worth giving visitors the disease. We could have cut down the avocado tree and removed the the problem, but the avos are awesome, and the tree produces hundreds every year.
Its a matter of prioritising. If we didn't have the groundwater to rely on (something many Australians don't have) the avo tree would have to go.
Keeping the gutters clean of leaf litter and sludge (from airbourne dust build up), and making sure they drain well removes another source of contamination. There are products available today that claim to keep leaf litter out of the gutters. Some sort of plastic mesh will probably do the trick as well.
Also a firstf flush system is a good idea. About 1 to 2 mls of rain should be enough to clean the roof of crap. A fit flush system is a storage area that fills before allowing water to enter the storage tank. It should have the worst of the gunk in it.
There a diagram of one here. You can probably rig up your own, or if necessary buy one somewhere.
http://www.rainharvesting.com.au/first_flush_water_diverters.asp
About 20 to 25 litres should do the average roof. I think you want 1 to 2 mils of rainfall to really clean it, and its about 1 suare m by 1 ml to get a litre, but check those figures.
What else? Materials... Plastic, Fibreglass, and Steel are the best bets, all treated to food quality standards. AFAIK the only manufacturers of rainwater tanks are in Australia, as we seem to have the worst groundwater in the world. I wouldn't know where to get any in the US. At the moment there is a backlog in Australia too, cos of the drought and a rebate on water tanks that the government has introduced, so you might be waiting a long time for one from here, not to mention the shipping costs. perhaps the best bet would be to get one fabricated out of steel, with a lining inside (the PDF goes into details on the sort of lining.)
That is the basic idea, but read the first link to get a thorough ide. Its a lot clearer and more orderly than what I have posted.
There are a couple more links here that are worth going over. They are from the US so should be more useful re local information.
http://www.harvesth2o.com/
http://www.harvesth2o.com/faq.shtml
I know this is a bit higgledy piggledy but read the links and if you need any further info feel free to ask.