You might want to experiment with your thinners - if you don't thin the paint, you're going to get streaks (paint won't flow together behind the brush), plus you get a thick layer of paint that quite often fills and eleminates fine detail.
Too thin and you again get streaks because less pigment is deposited. So what you need is to find the exact amount of thinner required to allow the paint to flow together behind the brush stroke. We're not talking a large amount (ie percentages) but two or three drops of thinner at a time untill you find the proper amount of thinner.
The above works very well for darker colors, but lighter colors (whites, greys, yellows etc) usually continue to leave streaks - for me the best way is to apply 2 to 3 coats layered very thinly. I usually apply the first coat painting in one direction, let it dry completly, then apply the second coat 90 degrees from the first and if a 3rd coat is required, let the 2nd coat dry completly and then paint in the same direction as the first. (example - first coat span wise, second coat, chord-wise and if required, 3rd coat spanwise).
One note on thinners. I use enamels almost exclusively (for both brush and airbrush). One thing I have noted is that a laquer thinner will work much better for thinning to eliminate brush strokes than the mfg's thinner or a generic thinner (ie minerial spirits, enamel reducer/thinner). So when I'm brush painting, I use laquer thinner exclusively.
Hope this helps a little.