I noticed working on a cintiq feels a lot more natural when sketching, so thats something to consider. Also, Krita (Krita.org) with the sketch brushes (type in 'sketch' in the filter bar) makes sketching a lot more enjoyable. There's a happy accident aspect that comes with its brush engine.
if you have a concept or a sketch done on a side view of a gun model, sometimes its very helpful to sketch a design on side view, then use projection to project your designs or sketch on a model and then later on do some illustrations or model highpoly parts based on the sketches you have done.