Tomato plants grow well in containers, provided the pots are large enough and the plants are kept well-watered and fed. Tomato plants grown in an open garden pull nutrients as needed from the soil. In containers, tomato plants often need more frequent feeding, as the nutrients in smaller quantities of soil deplete quickly. Feed these tomato plants at least once every 10 to 14 days. This is assuming you are using liquid or granules, rather than fertiliser spikes (these are left in the soil).
Use a good starter fertiliser, mixed in with the potting soil, when you first plant tomatoes. If you've already planted them in containers, add a water-soluble starter fertiliser to the top 2.5 cm (1 inch) of soil and water it in well. When water starts seeping out of the container base, you've watered deeply enough. If you are using a liquid fertiliser, apply it carefully to avoid splashes and follow mixing directions and application directions fully.
Fertilise the plant again in two weeks. This time, use either another water-soluble liquid or granule fertiliser (for tomatoes) or add a tomato fertiliser spike to the pot. Fertiliser spikes release fertiliser gradually over time (up to two months) -- these may be more convenient if you have a very busy schedule. No matter which kind of fertiliser you use, water the soil well so the fertiliser travels down towards the plant roots where the plant can use it.
Continue fertilising the plant every two weeks (unless you use spikes), until the first fruit blossoms appear. When this occurs, start fertilising every week throughout the rest of the growing season.
Use fertiliser created specifically for tomato plants. Tomatoes have certain nutritional needs that generic garden fertilisers may not meet.
If plants are growing and bushy but aren't setting any fruit, you may be over-fertilising with nitrogen. Cut back on the fertiliser for a while or use fertiliser containing less nitrogen to see if this helps.
Always water tomato plants deeply and at soil level rather than overhead from a sprinkler.
Avoid getting fertiliser on the leaves of the tomato plant -- this may burn the leaves.