I tend to do all my X-Wing repaints by hand. I've seen many great examples of airbrushing, but at the scale involved, it's not really required.
A good trick to use, if you have an area to paint with a lot of detail (greebles, panel lines, corners) is the Modelmates rubbing black product. You basically brush it on, and before it's completely dry, you remove the surface excess with pieces of paper towel or cotton buds. What remains are the very fine panel lines and shading around the surface details.
With a bit of practice, this should come out as good and probably better than the X-Wing prepaint jobs.
Another angle is to use washes and then use a brush to clean up the panels, again leaving the black lines in the recesses.
Most painted on details can also be done freehand, such as squadron markings etc. Just use panel lines as guides and clean up any mistakes with your basecoat.
Oh; and use gloss varnish on the cockpit canopies/panels/windows. A simple black with some varnish will go a long way there.