Specifically with tomatoes, for a plant that has reached (roughly) 6+ inches tall, I will set the root ball of a plant on a 2-inch mound of soil in a larger pot (3-gallon size) and then bury the plant with soil, compost, or a mix of both up to the seed leaves. That usually only fills the pot 1/4 to 1/3 full with rich soil.
Let it grow a 2 or 3 weeks in that.
It will quickly root into that soil. After it has grown some more, bury the stem a little more in rich soil, another 2 or 3 inches.
More growth will happen. keep repeating the process. You will get a plant with a prolific root system.