Keep it simple. Juice 3-5 types of vegetables only and save most of your fruit for smoothies.
Keep it fresh. Nutrients diminish the moment you have juiced so juice as you need it and don't keep it in the fridge.
Keep it small. A small glass of nutritious vegetable juice will go a long way. There is no need to drink vast quantities of juice. Think about what occurs naturally - you would eat the whole fruit. So it is not normal for humans to drink a great deal of juice.
Keep it away from food. To avoid fermentation, take your juice away from food by at least 45 minutes.
My favourite blend is the following:
1 carrot / 1-2 celery stalks / 1 apple / 1 beet / small piece of ginger to get those digestive fires stirred.