This Mark Cuban quote (from www.innerworth.com) summarizes a solid investment approach.
I take my biggest positions in stocks that are complementary to other things I am doing.
Once you understand a specific industry you’ll find yourself connecting the dots to create more opportunities. My background is in unconventional gas so I’m drawn towards companies with big positions in giant unconventional gas plays. In my research it was obvious that oil and gas service companies would be very busy drilling closely spaced unconventional gas wells and that made for a solid complimentary trade.