Here's how I do mine...
1. I rub it randomly all over in the wheel wells of my car to pick up brake dust and grime. If that doesnt dirty it up enough, I rub it on the grates and inner lid of my BBQ.
2. I take it out into my yard (all dirt I live in the desert) and toss it, roll it and grind it with my shoe's into the sand.
3. I randomly mist it in area's where you would expect sweat build up in real life with spary paint (i usually use a grey primer, a brown/red primer and an od green). I LIGHTLY mist the crotch, butt, front and back of knee's, waist area, neck, underams. Those are the area's that will most likey be seen, yet still covered. I also make sure to hit the other area's of teh suit as well but, that's my primary focus.
4. I hang it in my garage for about a week to let the stains really set in
5. I wash the flightsuit on the gentle cycle in cold water. No detergent, just a littel fabric softener to get the stank out. I want the stains to set.
Mind you, it will not be AS dirty once it's washed as it is after you thrash it. No matter waht, some will come out in the washing machine.