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.
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.
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.