Imagine I am building a large display that will cover many square feet and have both tall buildings and lots of small details. What are some techniques for making these things modular? Is it safer to disassemble partially or plan to carry displays whole? What do the builders and clubs at shows do to transport large layouts?
I wouldn't recommend transporting anything larger than a 32x32 stud baseplate. Use Technic pins and 1x2 bricks to connect parts of your model in the same way as the Modular Sets have done. The technique works well for buildings, planes, boats and just about any kind of large model.
Remove as many small parts likely to fall off in transportation as you possibly can and place them in a sealed plastic bag or other suitable container. Place the rest of the model with the extra parts in a cardboard/plastic box. The box is essential for catching any other fragile pieces that fall off the model. You may also use bubble wrap or tissue paper to pad the box as much as you can.
Once your Lego model has completed it's journey allow yourself some time to fix inevitable damage.
This is a fairly novel idea, which I haven't seen done before. Flickr user Legozilla has built a model that is specifically designed with transport in mind.
This particular design is unlikely to be damaged in transport. The flatpacked version of this model is significanly more space spacing compared to its 'built state' where it's volume is probably 90% air.
Occasionally, we need to fill in a spot in a display because a member needs to be late to a show or some other circumstance. To meet that need, I have a couple of buildings that I tote around to fill such a gap. This set of buildings is modular, and collapsible so that it's easy to transport and flexible enough to fit in most corners of a layout. It's also fun to fold and unfold just to watch people's reaction.
Note that you will probably need to de-greeble your models first. Small parts that fall off are annoying, and there's few things worse than finding a sea of small parts in the bottom of your shipping box. A collection of photos designed to help with reassembly is likely to be helpful, and will be essential if you want to accept offers of help with the reassembly.
I tend to ship the more fragile sub-assemblies in dedicated shipping boxes with a foam cut-out shaped to hold them. Rather than plastic wrap I use plastic bags. The bag is just there to keep foam and Lego separated (finding a 1x1 tile among 6-8 pieces of foam is annoying). Try to keep your boxes all the same size and shape, or multiples theerof, so they're easier to pack. The way Lego does :)
A useful variation on Ambo100's folding building that I've seen once, was an invertable building. The extremely detailed front wall was removable, and the inside built so that it could be flipped round to have all the detailed bits on the inside. That way most of the "outside" when shipping was reasonably solid, planar surfaces.
I build Technic, so my constraints are different, but the other technique I expect to use shortly is custom plywood boxes and platforms. For a metre-square model I will build a platform to hold it, then make walls and a lid that can be attached either above or below the model. One way up it's boxed for transport, the other way it's got its own table for display. With a little care I hope to make the base exactly 6 bricks high, so if it needs to go on a table with other models it will be relatively easy to get the height right.
Modularize, modularize, modularize! If you are putting models in a train show or similar, think about building large models so that they can conveniently be broken down into modules of no larger than 32x32 studs.
You can use "Saran" wrap to gently wrap some models. It keeps the parts from falling to the wayside, but doesn't neccesarily keep the models intact during transit. Even if you box it up as well, there is always a good chance you'll need to touch up any thing you build then move.
I generally agree with the compartmentalization and disassembly for transport and the need for sealed transport containers already put forth. Some additional ideas that I find helpful:
I like to have a solid base for my creations and if I'm unsure of the tables available at the display site, I like to have a precut plywood piece to go under my build to allow some greater control.
Take photos and bring them with you to assist in reassembly. Or more over the top you can have your entire creation modeled in LDraw or similar software and bring a laptop with you. Pictures can really help to check and make sure everything survived the move intact and has been reassembled correctly, especially if you have assistants who might not be as familiar with the build as you, or if you find a loose pieces and are trying to figure out where they go.
If there are specific pieces that you know are problematic, bring some extra pieces in a separate container for that last minute fix.