There is a commercial product called an "Earth Box" that I highly recommend.
It's major advantages over conventional garden pots, is that watering happens from the bottom up, and the soil is enclosed in black plastic. The black plastic warms the soil and prevents weeds.

Everyone who knows me knows I'm cheap. Also they know that I like to make things myself. Here is a cheap substitute for the store bought product.

Step 1: Stuff You Need.

Materials

A three gallon bucket.
A plastic colander that barely fits in the bucket.
A 2.5 ft long piece of 1/2 inch diameter PVC plastic pipe.
A draw string closure black plastic bag.
Potting soil.
Bottled Water container.

Tools

7/8 diameter Hole saw
3/8 diameter drill bit (not pictured)
Razor Knife
Coping saw (not pictured)
(Metal shears -- not as good as coping saw)
Shovel, gloves, bare hands, depending on your level of comfort with potting soil.
Safety glasses, to keep stuff out of your eyes.

Step 2: Put Holes in Bottom End of the Pipe

Put some 3/8 inch holes in the bottom 4 inches of the pipe. Use a file or razor knife to remove the burrs from the holes.

Step 3: Put a 7/8 Inch Hole in the Bottom of the Plastic Colander.

We need a clearance hole for the pipe thru the colander.

Step 4: Remove the Handles From the Plastic Colander

The colander should fit snugly most of the way into the bucket.
I tried the shears first. The result was some cracking.
The coping saw worked much better.

Step 5: Fit the Colander Into the Bucket; Fit the Pipe Into the Colander

This part is called Assembly.
The pipe goes with the holes down, the colander goes dome side up.

Step 6: Add Soil and Plants.

The soil goes on top of the colander. For tomato plants, two thirds should be buried.
The remaining pipe will work as a tomato stake.

Step 7: Bag It!

Remove the handle from the bucket.
Place the bucket in the bag.
Draw the bag up around the plant.
(optional) Re-install the handle (by feel)

Step 8: (Optional) Create and Install a Funnel.

A plastic water bottle almost fits the pipe.

(Once again too cheap to just buy a funnel that fits.)

To make a seal, a couple of layers of bicycle inter tube worked well.

To modify the cap to fit the pipe, the hole saw was used without the pilot drill.

The sides of the cap made a good guide. the clamp kept the cap from deforming itself out of the vice.
The pictures should be enough for someone as cheap as me to duplicate the process.

Step 9: Add Water

(not shown) Using a marshmallow roasting stick to use as a dip stick, add water to a level of 6-8 inches.
Add water, and occasionally liquid fertilizer throughout the growing season.
I look forward to enjoying the fruits of my labors, as i have in previous seasons with a similar, but more complex design in years past.