I used to be part of the "bird's nest brigade" but have been getting progressively neater with every plane I build. These days once I've sorted out where everything's going to go, run it neatly under servos trays or around formers etc I use regular old cable ties (aka zip ties) every 3-4 inches to form a bundle (a bit like a car's wiring harness), then use blobs from a hot glue gun to glue the bundle to the airframe at strategic locations. I also use a blob of hot glue on any servo connections that're going to be permanent (eg: extensions running through wings).

The hot glue won't move on its own, won't vibrate off, and isn't affected by fuel etc, but can be easily peeled off if you need to make changes, and the cables ties are also easily cut off and replaced. Bundling them al together with the zip ties makes it fairly robust so it'll hold whatever shape it's bundled into.