Ahhh Juicing,
Ive worked @ the Juice bar at health food stores for a few years, so i have some experience in this field.
As to cover what hasnt been answered,
if you want to preserve the juice for a few hours, citric acid from a fresh squeezed (only) lemon will preserve the juice but know that its always better to drink it slowly and straight from the juicer immediatley.
You dont need to spend 200$ + on a juicer. the 20$ ones are fine if its just for you (and perhaps 1 other).
Clean the juice extracting basket with a firm tooth brush and water before the beta carotene stains dry up.. if they do, then you have to use a brillo pad..
For health food stores, Santos is the industrial standard, its an 1800 dollar juicer used for high volume juicing.
My favorite for circulation and flavor is a Carrot apple Ginger (mass ginger (3-4" piece if you can take it)
with lemon ..
as for the green leafys (spinach, parsley, kale, watercress etc..) there isnt much juice in them, so i say its better and cheaper to just eat them raw. with the juice. And the correct way to clean leafys is to float them in water till all the dirt is gone.
(curly parsley is my fave)
When it comes to Wheatgrass, know that in one oz of juice is enough vitamins and minerals to cover you for the day when it comes to nutrients. BUT you cannot thow wheatgrass into a normal veggie juicer. it will waste it. you need a pulverising wheatgrass juicer, heres one..
http://www.discountjuicers.com/healthyjuicer.html
always buy it fresh, and if you dont feel like you want/need a WG juicer, you can just chew the grass (which is alot more sensible in my opinion) but you have to chew a nice 1 1/2" bundle.
other wise go to the healthfood store, pay the 1.75 for a shot.
Wheatgrass and raw alfalfa (or any fresh) sprouts are LOADED with everything.. every vitamin you need and then some.
if you have any questions id be happy to answer.
(chefs trick, cut the lemon in half, cut into the center of the exposed lemon and with the knife pointed at the juice, you can squeeze the juice down the knife into the juice! no lemon spatters, all in the juice, and lemon gives you the C and will cut the spice of any garlic or ginger you would add to the juice!)