While it can be entertaining, try not to get too caught up taking over the gyms of rival teams. Rather, make sure to strengthen your own team’s gyms by choosing the ‘Train’ option. This allows you to battle Pokemon that are stored at a gym by your allies. Doing this raises a gym’s Prestige Points, which increases the number of Pokemon that can be stored there, thus making it more challenging for rival teams to take it. In addition, training raises your XP, regardless of whether you win or lose, so long as you defeat at least one Pokemon, and Pokemon won’t won’t need to be revived if they’re defeated by a friendly trainer.
Originally Posted by: gamespot.com