They are tough to get right, but once you develop your technique you'll be fine. The whiteish/matte marks you're leaving are most likely the thinner you're using.
For me, I apply a sealing coat over my paint--typically Vallejo Satin Varnish, or Humbrol Gloss Clear. I let that dry for at least 24 hours then do the streaking. I use the AMMO product straight out of the bottle and apply dots and lines where I want them. I then let that dry for at least an hour. Next, I use Gamsol--an odorless mineral spirit you can get at an art supply shop. It's very mild, and doesn't seem to damage my sealing coat, nor the base paint. Anyhow, dip your brush (I use a soft flat sable brush) in the gamsol, and wipe off about 50% of it. I make ONE drag down, let the AMMO soften up a little, and then ONE drag up. If this doesn't give me the results I'm after, I make another super light drag up and call it good. Don't be afraid to let it dry and apply another layer.they all blend in and get kind of "grimy".
I have also used Tamiya's X-20 (not X-20A) and come out with some good results, but you have to work very carefully with it as it will eat your sealing coat and you'll start to pull off paint.
If I can be of any more help at all, please let me know!
Here's some very light streaking using AMMO's dark streaking grime and Gamsol (if you look at the inside of the left arm, you can see where I applied the effect, and forgot to streak it before sealing it):