Proper lighting, environment, and nutrients are the best ways. Beyond that, there are a whole lot of theories: shocking the roots with ice water, complete darkness for a few days before harvesting, cold night temps, and a whole host of nute supplements.
Most of these are anecdotal and don't coincide with any real evidence, but some growers swear by them. You'll have to read up, try to parse all the BS, and decide what you want to try if anything.