Although not guaranteed, one tip for possibly getting upgraded to 1st class is to dress like you belong there. Yes, you can buy a 1st class ticket and dress however you want. But for an airline to upgrade someone to 1st class, they typically shy away from people dressed too casually (think shorts, flip flops, and tank tops).
A co-worker and I had a business meeting at our corporate office. We were dressed in suits. To save expenses, our tickets were for economy, as the flight back home was less than 3 hours. When we got to the airport to head home, we were both upgraded to 1st class (my one and only experience there). Our time to get to the airport and make our flight was so short, neither of us had time to change or even take off our ties. I guess it paid off, though.