I really don't think I can match with some of the very detailed works already here, so I usually make simple, practical (such as suitable for certain challenges or combat) planes that other players can use well.
Simple, interesting, practical, that's my motto as a SP creator.
Currently using following building techniques:
Zero Control Surface Flap: using a rotater to change the angle of a wing section instead of control surface to have the effect of flap
Tub Cockpit: submerge the cockpit camera into the cockpit section of hull, then surround it with a "tub" to make interior
Unexposed Fuel Tanks: submerge another hull block inside the hull to make sure fuel-containing sections are not exposed
Cosmetic Wings First: making cosmetic wings before the actual effective wings to simplify the process
Onboard Fire Extinguisher (basically failed): attach a minus-damage gun to fire at damaged sections to repair it, but it can't put out the fire nor stop it from exploding