There are a few things you can eat/drink that won’t break the cleanse. Any fresh fruit or vegetable is acceptable and will help you feel full. You can alternate the fruit or vegetables with your juices or consume them at the same time.
You should also drink regular, lemon, or cucumber water throughout the day to hydrate. During a juice cleanse your body is hard at work removing toxins, burning fat, and repairing damaged cells, and it is common to become dehydrated (which causes hunger and possibly headaches).
Also, here is a blog post that explains some options if you do not want to do a strict juice cleanse.