This post is intended as a response to a question from a friend.
I should say at first that plants should not be put in pots unless there is some reason they cannot be planted in the ground. Nature never put a plant in a pot. This means that containers are second choices in terms of location, and the reason for this is that plants do not generally do as well in a pot as they do in natural soil. There are exceptions and there are of course reasons to plant things in containers--reasons unnecessary to go into here.
The first thing to consider when planting in a pot is the restriction of the container. Plants like to spreak their roots far and wide if they can--a typical tomato will absolutely fill a three-gallon container with roots in two months' time.
Container plants lose water rapidly. This is because moisture departs upward, downward, and through the sides in most pots. Be aware that this means much more frequent watering--and sometimes watering from both bottom and top, but only after establishment and firming of the soil.
Now, about that method of planting. First, seeds. Fill your pot with whatever planting soil you intend to use. For illustration, imagine that it is a typical three-gallon nursery pot. Shake the soil down so it settles. Now empty that pot into a much larger container such as a garden trug or a five-gallon paint bucket. Add at least another third again as much planting soil. Now here comes the important part: add water.
Using your garden hose, saturate the soil in the container completely, until it resembles wet mortar. Be sure it is wet all the way to the bottom. If you have made it too wet, say, like pancake batter, add more soil. When you have done this, pour or scoop the wet soil into your potting container. Of course you should have partially blocked the drain holes with pebbles before this.
Let the soil sit for a few minutes. You should see water seeping out the drain holes of your pot. The wet soil will settle. Add more wet soil until the soil level is mounded slightly above the rim of the pot. Now, using the instructions on the seed packet, plant your seeds in the wet soil. Push them into the wet dirt mixture. By the next day your soil will have settled yet more; you may add a bit more dirt. Save the extra soil mixture--you will need it when the level goes down further.
Potting an existing plant: Do the same as before, making a loose wet mixture of your soil. Remove your plant from its present pot. With your hand, measure the height from root bottom to the soil level. Dig a hole not quite as deep as that. Insert your plant and add more wet soil until you've mounded up the dirt a bit higher than the rim. If by next day your plant is too high, push it down gently; if it is too low, use a pair of spoons or small garden shovels on either side to clamshell-lift it to proper height.
Why this method works: When people plant in pots they typically fail to moisten the soil sufficiently, especially further down in the pot. The plant roots find no moisture below and the plant suffers. Watering by soaking the pot so that the moisture rises is helpful with an established plant but with a newly planted item often the soil will simply wash out the bottom via the drain holes.
Depending on your plant it is also a fine idea to add small amounts of any soil amendment to the wet soil mix, being careful not to add so much as to burn roots. You have to know your plants, in other words. If you aren't sure, just go with a good planting mix and/or aged (I emphasize aged, which means composted) manure.
Try this method with one tomato plant and use the traditional method with another. I guarantee your wet-mortar tomato will produce more fruit.