We have done this a couple of times. Step number 1 - take plenty of photographs, especially of the joints and corners, and the way the door hangs or slides.
Step number 2, remove the glass. If you possibly can, get hold of a couple of these:
http://cgi.ebay.co.uk/55mm-Mini-Suction-Cup-Dent-puller-Power-Grip-Glass_W0QQitemZ290204130470The glass will be held in (probably) with little W shaped wire clips. You may also find that the sides of the glass are held in with rubber-backed strips which are screwed to the frame. You obviously need to remove the clips and loosen or remeove any securing crews.
Start with the roof. It's best to work as a two man team, with two stepladders. One inside the greenhouse, one outside. Wear leather gloves at ALL times (I still have the scar). Use a suction grip plus one leather-gloved hand to hold the glass and carefully remove each sheet and stack it on edge against a wall.
Then remove the door and the glass from the walls, which will leave you with a bare frame.
Step 3. The roof pole will be the first thing to remove from the frame. It will be secured with screws and some little patent bolts like this:
http://cgi.ebay.co.uk/10-Greenhouse-Cropped-Head-OR-Square-Head-bolts-nuts_W0QQitemZ330209113338It's worth buying a few of these anyway - you're bound to break or lose some. They become brittle with age.
Then it's just a matter of undoing the frame top down. You'll find that a number of the spars are identical, so bundle these together. It's worth using a marker pen to assign a letter or number to the different bits and keeping a note of what's what.
Reassembly is, as they say in the Haynes manuals, the reverse of dismantling. Just far more fiddly. Obviously make sure that you have a nice flat base for it.
I've assumed that it's a metal framed greenhouse, by the way. Wooden framed ones are a different pot of begonias altogether.
