A pair of these on front and rear racks and I can carry a huge load.
Veggies from Haymarket, drinking water from the reverse-osmosis filter at the food coop, etc.
Step 1: Milk Crates, Rope, Drill
Just put a loop of rope through the top of the crates and set it over your mountain bike's cargo rack. I drilled holes through the edge of my crates to get it to ride a bit lower, but that's not really necessary.
If you have an extra pass of rope at one end, you can hook it over the lip at one end of your bike rack so it won't slide off.
By the way, why in this supposedly developed country is there so much perchlorate (thyroid problems) and other crap in the water? People are paying more for bottled water than gasoline. Are we going to invade Canada next?