I like to draw blueprints of my kits in AutoCAD. That doesn't mean you cant draw stuff by hand. I like to make side, front/back, top/bottom views of the pieces I need to make. I print out the blueprint on craft-like sticker paper. I cut the design out of the sticker paper using scissors and stick the design on the plastic I'm going to cut my pieces from. I use my table saw or nibbler to follow along the design to get my desired shape.
For square and angular pieces, it's hard to beat a milling machine. With a mill, you basically remove material from a chunk of materal like "butter board". This is one of the techniques Captain America I saw use so I try to do the same thing.
To me the harder but more economical way to make square/angular pieces is to build a box out of flat sheets of plastic. This is simular to how paper models are constructed but in our case we use sheets of styrene instead of paper.
For surfaces which have compound curves, it's hard to beat good old putty/clay. A good sculptor can make extremely convincing reproductions. Machines just add a level of acuracy and repeatibility. Me being an engineer with no sculpting ability, I prefer a solid 3D computer model and a 5-axis CNC router or a stereo-lithography machine. All of that is super overkill for our hobby but that's what i do for a living. The hard part for me is finding 3D solid models of Macross stuff. It takes a lot of time to draw this stuff on CAD software.
I find most of my scratchbuilding inspiration and techniques from MW and fine scale modeler magazine. I'm a mechanical engineer, so I also know/do a fair amount of machine shop and rapid prototyping work, so blueprints and hi-tech are second nature to me. I just need to make more time for my beloved Macross/model hobby.