1. Sounds obvious, but make sure the company you order the greenhouse through (if not buying second hand) has taken the order correctly. Email/write to them to confirm if necessary. And read that bewildering invoice/order sheet carefully, getting them to talk you through all the abbreviations if necessary. Also, find out what role they play if there is anything broken, what fees there are if a redelivery is needed etc (i.e. think about what can go wrong).
2. Don't open all the boxes and spread the bits out as soon as they arrive to check them off the list of bits. Do, see if they've been bundled into some kind of order (i.e. side wall, end wall, roof) and keep the bits together. Then check them off.
3. Check if the glass is intact and what you ordered. Again, sounds obvious but a friend had a pack with broken glass all the way through and another with bits snapped off the corners.
4. Phone a friend. You really do need help putting it together. When putting my frame up there was only two of us so we used bits of wood to keep the sides propped up until we had screwed them together. It also helps with glass, especially on the roof.
5. If you can, buy large pane toughened glass (or I presume polycarbonate). Not only is it safer, but it is easier to put up as you have less glass to install (1 sheet for every 3 on the wall or 1 for two on the roof). It means less clips as well (which reminds me - wear gloves or something, two weeks after the last one my thumbs still hurt!).
6. Masking tape is really useful to temporarily hold bolts in place - especially when working on the roof (see photo).
7. Alway put extra bolts in the bolt channels. For shelves, hooks, or just those bolts you forgot to insert. Tape them in place as well.
8. Think about access. Do you need ladders, or to hire a platform or something? And if you are putting it in a corner and can't access it from the outside do you need to change the assembly order?
9. It takes longer than you think! If you count everybody's input around 35 hours work went into my greenhouse by my three assembly teams [making the sides, putting frame together and then glazing/finising] or 14 hours overall. It will take longer than that if you work alone for some or all of it.
10. So far, I think that concreting it into the ground has been the easiest and securest way of securing a greenhouse. Doesn't take much time (as long as the site is level) and is very secure (my neighbour's greenhouse blew over).
I think that's it for now, I'll edit the post if I think of any other tips. Please feel free to add your own in the comments section.
Having said all the above - I thoroughly recommend having a greenhouse of any variety (know of some home made ones that are very effective). Not only is it good for tomatoes and cucumbers but you can grow seeds, potter about on cold/rainy days and use it to collect rainwater!