For me, the key is to ride, ride, ride and enjoy it.
I find simple and informal interval training useful. When I feel like it, I gradually apply full power until I begin to deeply regret it and then slowly ease off back to cruising speed, repeating as the spirit moves.
I've also found that a longish warm up -- trying to ride sort of slowly and spin gently for the first five miles -- results in faster rides. Starting that way I don't even decide when it's time for regular riding. It just happens.