Go slow. That's the best distressing tip of them all. With that rule of thumb in mind, get yourself some acetone or rubbing alcohol and fine grit sandpaper and go for it! When rubbing the leather with acetone, the dye is either removed or moved around, to a certain amount. Sanding on and around the seams highlights the areas and give a worn look. Don't go too coarse on the sandpaper and mind the threads - you don't want to sand them off... also, once you've sanded, be careful with the acetone in those areas. Rubbing the sanded parts will move dye over to them, re-dye them and your sanding effort is ruined.
Pause often. Otherwise, there's a risk you'll go overboard and the result looks like a thrashed leather jacket instead of a distressed one.
Remember that by sanding off the top layer of the leather, you also expose the core and make the jacket more vulnerable to rain and dirt. Treat the jacket with a quality leather dressing, such as
Pecard.
Remember, easy does it. Also, reference pics are good to have around. Soaking the jacket a couple of times will remove the "new jacket" sheen. Sleeves also wrinkle nicely when wet.
In time, your jacket will go from this:
to this: