Important things to remember when using rainbarrels are:

Do not use the collected rainwater for drinking, cooking, or bathing.

Set the rainbarrel on firm, level ground so that it won't tip over. 
It may be helpful to use sand or bricks to provide a level setting. 
If you set the rainbarrel on an elevated platform make sure that it 
is secure.

Do not connect the rainbarrel to your downspout if you have recently 
applied chemicals, such as moss killer, to your roof. 

Do not connect the rainbarrel to your home water supply.

Do not install a rainbarrel where it could be used to allow an 
intruder to gain access to your home.

If your downspouts were connected to the storm sewer system, make 
sure they are reconnected during the rainy season or whenever your 
rainbarrel is full.

Rinse the inside of the rainbarrel occasionally.

Try to cycle the water through the barrel as often as possible.

Contact your local water bureau or city water person and see if they 
have kits, kickbacks or something. I looked online and for areas it 
is cheaper to get a kit through them than going out and getting all 
the stuff for one..

Links:
http://dnr.metrokc.gov/wlr/PI/pdf/cistern-water-saving.pdf
http://dnr.metrokc.gov/wlr/PI/pdf/covington-rain-barrels.pdf
http://dnr.metrokc.gov/wlr/PI/pdf/Rainwater-Harvesting.pdf
http://www.rdrop.com/users/krishna/
http://www.yougrowgirl.com/garden/rain_barrel.php

tenzicut - who will have one once she has a house