When we harvest basil, we usually take about half of the current top growth, then fertilize and wait for it to regrow before harvesting again. We can often get 3-4 cuttings from each plant this way during the season, and since we have several plants growing at one time, we can usually have a nice bunch of basil any time we like since we don't cut them all at once.
In general, it is true that when harvesting a perennial herb, you don't want to take more than 10-20 percent of the top growth at a time - but basil is an annual and by midsummer it is always trying to go to seed. Once the plant begins to flower, more energy goes towards the process of setting seed and much less into producing leaves, which is what you want it to do. Cutting basil back hard usually results in the plant going back into vegetative growth mode for a while, and staves off flowering for that much longer.
If you don't need that much basil at a time, then on the branches you do cut, cut them back by about half. Be sure you leave some leaves to keep the plant going while it recovers. If you do this all over the plant as you harvest for kitchen use, it will help the plant to stay in vegetative mode. How often you do this depends entirely on how well your plant is growing.