When you look at any period photographs of paratroopers in combat kit, they are always dirty; the kit is marked, stained, repaired in places and has been heavily worn. Assuming you're not going to portray a fresh-faced trooper who is going to be based stateside and always be in training then your kit should reflect the appropriate amount of wear and tear.

So, what is the best way to make your new uniform look like a battered, combat worm uniform... simple, follow these steps carefully:
1 - put on uniform correctly. Trousers on legs, jacket on torso.

2 - wear uniform at shows
3 - extra steps - to speed up the process, wear the uniform when doing gardening, working on your car, around a building site, when fixing the shed... in fact wear it anywhere; anytime you're doing anything will age and wear it in naturally. 

Do not try and "fake" dirt, wear and grim... it will look fake! Let it happen naturally and your kit will look amazing when it does. Be patient!