I love juicing... I love it even more when friends get a juicer and start demanding recipes of me :)
Since I tend to just throw things in at random, the request sets me perusing through various resources where I inevitably find some very yummy concoctions. Here is a really good one for before a workout as the beets, apple and carrot will provide your body with a kick of natural sugar, and the cucumber and celery will alkalize your body and diffuse some of that sweetness.
1/2 an apple
1/2 a cucumber
6 celery stalks
Chunk of ginger (if you like the idea of 'spicier' tasting juice, one with some kick, up the ginger)