For whatever reason, it took me close to a decade to get serious about my painting techniques with miniatures, but now I have, and it took me far too long to figure out how to work with thinned paints.
First off, pretty much every line, regardless of the hype, has paints that are a little too thick for use straight from the container. Personally I'd prefer this was the case, in fact, since I can by thinning materials far more cheaply than I can buy the paints. Right now I am trying what is alleged to be Jennifer Haley's thinning mix: 1:1:2 retarder-to-flow improver-to-water. Usually I'm finding, for base coats, 1:1 works reasonably well with P3, Vallejo, and Reaper Master paints, and for tinting coats, around 3:1 works well, but at this point it's much more dependent on the line or even the individual paint. I've tried washes at very low paint ratios (10:1) and have found it almost too thin, but I'm still working on my patience.
Next, once you have your paint, what to do with it. At first, I had the biggest pain working with these thinned paints: I couldn't control the flow, they'd sweep into crevasses or over other surfaces abruptly. Eventually I became conscious of exactly what I was doing wrong, however, when I watched my subconscious behaviors working with the normal paints. I frequently use the edge of my painting station to wipe excess paint off the brush after dipping it; the reduced paint volume allows for increased control. Once I'd made this connection, I watched the spread of the thinned paints as I stroked the brush against that surface and could easily see that it would start off bleeding every which way, but within a few strokes became controlled. So for now, every time I load the brush, I watch the area I brush until the paint gets to the control level I want.