Notice if they are being eaten or if they are starting to look a little yellow. Check for symptoms that may be corrected before they kill your plants. It does not take long for a problem to get out of hand if neglected. Identify the bug you found crawling on your plant to find out if it is harmful. A good book on garden pests common in your area is a wise investment.
Any organic soap is a good deterrent.
Because it is organic, the soap will have to be applied more often than chemical pesticides. A light watering every day is preferable. This ensures your plants will not be water-stressed which can cause tomatoes to split. It is best to water early in the morning, or late in the day - never in the heat of the day when water will evaporate immediately. Or, buy a balanced combination of all three and use once a week.