As with most endeavors, I've found:
Success comes from good decisions.
Good decisions come from knowledge.
Knowledge comes from experience.
Experience comes from bad decisions.
Kind of a twist on the trial and error thing, but that's how I've been doing it (and paraphrased from the original, of which the source I cannot remember)...
No other trick or tool seems to help other than losing money an a few houses and learning the hard way.
That is my experience.
<results may vary...>
