Only once, really. Starting in Pokemon Diamond, I had a training regime set that I've followed in every game since. This way, I can sweep through the gyms and take on the Elite Four without worry. However, the training regime can be very annoying.
It's simple: All Pokemon in your main team to level 15 for the first Gym, 25 for the Second, and add five levels for each gym afters, and another five before fighting the Elite Four. If I remember correctly, you'll be either Level 55 or 60 when you fight the Elite Four. Get a few potions and revives, and you'll be good to go.