Tomato plants have a reputation for being self-pollinating. While it is true that they have both male and female parts located inside each flower, the pollen still needs help getting where it needs to go in order for the plants to produce tomatoes. In outdoor gardens, bees and/or gravity are usually only too happy to do this job for you. If you are growing tomatoes indoors or in a greenhouse, however, this may be a task that you need to perform yourself. Gravity will help, but if the weather is particularly humid, Mother Nature will need your assistance.
Observe your tomato plants and their flowers. If their flowers are open, you will be able to pollinate them.
Blow gently on your tomato flowers and watch. You may see pollen flying around, which is good. Try to blow downwards to encourage the pollen to go from the stamen of each flower into the pistil.
Tap the stems of your tomato plants with a small round paintbrush or chopstick. This will cause vibrations. The pollen will be knocked loose and should be able to let gravity guide it to its destination.