If your herbs are growing bigger you might want to use some parts of them or cut them back to ensure a "bushier" growth. Here is how to do it:
You can start pruning if your plants have at least 2-3 pairs of real leaves.
Be sure to leave at least 1-2 pairs of real leaves on the plant, otherwise it might die.
Cut the stem with sharp scissors right above the node (the place where leaves come out).
From every cut 2 new shoots will appear and your plant will grow again.
This will extend the lifespan of your plant greatly.
Easiest plants to practice on are the basil, thyme and lemon balm. But most of our herbs can be pruned. Practice makes perfect!