Greyhound doesn't work well to Hawaii
In all seriousness . . . Depends.
If you're looking at a large fuselage airframe, (like a carbon-z), you're pretty much stuck with freight, Greyhound, being an excellent low-cost option in the contenetal US (pack durable, and consider using a crate)
If you're looking at something with smaller fuselages like gliders, many still pack long, but the width and diameter doesn't have to be. They ship like snowboards and skis - box them up in a hard case and under the plane they go . . . But even that will depend on the airframe -- many will have wing panels that separate and detachable tail feathers, breaking the plane down to regular luggage dimensions. Pack in a hard case and slip it under the plane.
If you're scratch/plan building such a plane (say, an FT storch or similar size), mod the design -- there are ways to make wings foldable or that break into panels, landing gear can be modified to detach, and even tails can be built to pop off the airframe. Into standard hard luggage and under the plane it goes. Each plane has its own challenges, but thats half the fun of building it yourself.