If you got this plant from the supermarket, it's worth re-potting it. you'll find you actually have lots of little seedling basil plants all squished into one pot to make it look like a bit fat healthy one when you buy it. If you split the seedling up a bit into a few new pots, they'll go back to being bigger leafed, and last for ages, especially if you just pick out the top two or three leaves for a while - the plants will bush out nicely. Basil likes plenty of water, but don't let it sit in a puddle of water and it prefers to be watered in the morning, otherwise the drop in temperature over night, combined with damp soil, can affect growth - just remember it comes from warmer countries, so treat accordingly.