When juicing fruit, stick to citrus, apples, pears and maybe melons. Berries, stone fruits, and tropical fruits like bananas, papaya, and mango are better blended into juice to make a smoothie. 

Watch what you combine with citrus...apart from carrots and apple, it seems to be to be the hardest to combine with other produce. But a frozen banana + fresh OJ=heaven!

Do not juice carrot tops, and go easy on beet greens. The former is poisonous and the latter can induce a cleansing reaction which is rather unpleasant (though they are exceedingly nutritious!). 

Don't forget lemons, limes, herbs, ginger, garlic, and turmeric...these are all great additions to a juice. You can also add a pinch of salt or a couple dashes of spice (e.g. curry, cinnamon) to add flavor and variety.