It's a HUGE topic that could span at least a couple years worth of material (And the cult of WWG must keep some of our "precious" secrets
). It really depends on what medium you are most comfortable with and what the desired effect is. For some the best solution might be to take a more hands-on approach and physically draw and build their own creations to later be scanned and tweaked digitally.
I myself don't really have one technique. I bounce around between conceptual design on paper, to Photoshop and once in awhile in my fav 3-D modeller (for perspective reference mostly). EVERYTHING starts on paper with good conceptual design and I think that's true for all the model designers here. As part of your conceptual design process surround yourself with good reference material! The best advice I can give you though is to wrap your brain around Photoshop. I've been using Photoshop for years now and I'm still learning new tricks that keep the program fresh. I was a graphic artist before all of this but once I got into Photoshop I found myself hanging up my pencils for a stylus. There are some amazing online tutorials out there for PS which are a good place to start but I would strongly suggest taking a PS course if you can afford it.
If you're looking for a "quick and dirty" approach to building models and texturing, I'm afraid your models will look well...quick and dirty. There's no substitute for patience and knowledge
