Well, the first thing to note is that the ones I'm growing as cordons (i.e. the "Indeterminate" ones) are already getting quite big, and it is becoming difficult to manage them with the pots arranged in a double row:
With my restricted mobility, it is difficult to reach over to the back ones whenever anything needs to be done to them, so very soon I will be re-arranging the pots into a single row.
One of the most frequent tasks that needs doing is the removal of the side-shoots that grow in the leaf-axils, like these:
If you don't remove the side-shoots, they will grow into lateral branches and the plant will get very straggly and hard to maintain. Furthermore, less energy will be available to the main stem. I always pinch out the side-shoots when they are still very small, using my finger and thumb. Once they get bigger it is necessary to use secateurs. If you like, you can use the removed side-shoots as "cuttings" and grow new plants from them. I don't generally do this, but I might have a try some time, just for fun.
Another task that needs doing is tying-in the plants to their supporting canes at frequent intervals, to keep them upright. At this time of year tomato plants grow very rapidly and they need tying-in about every other week. I use soft garden twine for tying-in, and I usually do a tie every six inches or so, in order to provide good support for later on when hopefully the crop will be heavy. It's good practice to make at least two loops of twine each time, thus spreading the load, since a single loop could easily cut into the surface of the stem.
Inadequate and irregular watering is the most common cause of failure in amateur tomato-growing. This is why I have got these so-called self-watering containers:
They have a reservoir in the base, which will hold enough water for a big tomato plant for a couple of days even when the weather is warm. Even with pots like these it is a good idea to check the moisture of the compost very frequently - I'm talking about at least once a day - and add water if necessary. In high Summer I often water my tomatoes twice a day. Constant dampness is much better than a cycle of drying-out followed by flooding! Erratic watering inhibits the plant's ability to absorb nutrients (especially calcium), which is a major factor in the incidence of Blossom End Rot.
Tomato plants are frequently grown in commercial multi-purpose compost, which often contains strictly limited amounts of nutrients, so once the plants start producing flowers, it is usually time to start feeding them. I'm hoping that mine will not be lacking in nutrients this year, due to all the good stuff I put into their growing-medium, but I shall be feeding them nonetheless!
My tomato plants benefit from two sources of feed. The first is a weekly dose of Tomorite commercial tomato-fertiliser, used as per the manufacturer's instructions (i.e. diluted in water). The other food they get is my home-made Comfrey Tea. I just put a load of Comfrey leaves and chopped stems into a bucket, cover them with water and let them rot. Within about 2 - 3 weeks they will have "deliquessed" into a thick green sludge. This sludge may be less than appealing to us humans, but it contains nutrients that tomatoes and other fruiting plants (such as chillis and aubergines) love - particularly Potassium. When the "tea" is ready, I dilute it about 50:50 with water and just pour it into the tomato containers, preferably just after normal watering has taken place, so that the Comfrey Tea is more easily absorbed by the compost.
So far, I have been writing about the Inderterminate varieties of tomato, but the culture of Determinate (aka Bush) varieties is different. Since they don't grow tall you don't need to do any tying-in (although I do generally tie them in to short (2-foot) canes for greater stability), and since you want them to be bushy you don't remove the side-shoots. However, what I have said about watering and feeding still applies. By their very nature, Bush tomatoes are going to be straggly and cover a lot of ground, so it is a good idea to think carefully about where you site them. Of course all my Toms are grown in containers, so it's not too much of a problem for me, and I can move them around if necessary. Tomatoes grown in the ground are a different matter!
One other thought: sometimes you see tomato plants bunching-up their top leaves into a tight ball (particularly overnight), with the leaves curling inwards towards the stem, like this:
At first sight this may look alarming, but actually it's fairly innocuous. It's caused by the plants getting too cold - or at least colder than they would like. The action is just the same as that which we humans do when we're chilly - we wrap our arms around ourselves and hug ourselves. When the temperature warms up they will return to normal.