You can cut back the leader of the plant to limit it's height, but it may impact on the amount of fruit it produces. Blossom Set is a good idea if your tomatoes are the only ones growing in the immediate area or if the weather is so cool that there aren't very many insects to help pollinate the plants. This will vary according to the variety, temperatures, amount of sun and general care. There are several things you can do culturally to help. Keep your tomatoes as evenly watered as possible, water from below, apply plenty of mulch, avoid growing tomatoes where you have had problems before and choose varieties resistant to diseases.
Tomatoes are a favorite of aphids and slugs.
Watch the new foliage for insects and spray with insecticidal soap.