Robert Heilbrunn's potential to know a bargain when he saw one became apparent when he hired Benjamin Graham to advise him on his portfolio for $25/month. For this sum, Heilbrunn was able to work with Graham and understand his methods of valuing various securities. Together, they bought a slew of bonds and stocks trading at large discounts, and subsequently sold them at their intrinsic values for a profit.
Heilbrunn later made an innovation to the field that is often used today: focusing on where a single stock trades in comparison with itself! Usually, analysts compare a stock's P/E or P/B with other companies in its industry or index. This can often hide stocks trading at discounts if the entire industry or index is depressed. Heilbrunn, therefore, was interested in what a stock's P/E (among other things) was in comparison to its historical ranges, rather than just against other companies.
Later, Heilbrunn invested his money not in companies, but in other value investors. He was able to retire comfortably by investing funds in Warren Buffett (only one year after he had formed his partnership), Walter Schloss, and Graham himself.