Almost a month after Pokemon GO's release you've probably been inundated with more tips, tricks and secrets than you can actually process. Here's a simple one for you: if you're serious about the gym game, don't get rid of your low level Pokemon straight away.
While it can be tempting to swap out all of your mid-level Pokemon for sweet sweet candy, especially when you obtain one of the same Pokemon but at a higher level, it can pay to hang on to them if you want to be the very best.
The reason for this is related to gyms' prestige points. You may have noticed that you get a different number of prestige points for your gym every time you train it (ie, battle your own gym to raise its level) but not many know the reasoning behind it. Here it is:
You get 500+ prestige points for beating a Pokemon with one of a lower level. That means 500 points per Pokemon you defeat, not for the entire gym. As soon as the Pokemon you're using is a higher level than the ones you're fighting, you only get around 100-200 prestige points for each Pokemon defeated.
What does this mean? Well, it gives you an easy way to level up your gym. If you've combined with a group of friends to take a gym, have the first Pokemon put down be a lower level (I use around 1000cp, half that of my strongest Pokemon). Then, train against this Pokemon with one of your Pokemon that's a lower level (in the high 900s, for example) but with a type advantage.
Ideally, put a fire-type Pokemon in the gym and challenge it with a water-type that knows water gun -- one of the strongest and fastest moves in the game. This means you can beat this Pokemon again and again, earning yourself a cool 500+ prestige points and leveling up your gym super quick. Even when other Pokemon of a higher level get put in the gym, you should only fight the first one. Beat the first Pokemon and run from the battle as soon as the second one pops up. You'll still get the points. Heal up and repeat for as long as you have potions to spare.
This tip comes in handy when you're actively trying to protect a gym from attack -- which can be hard, when a successful attack can wipe anywhere upwards of 2000 prestige points from the gym. If you have a couple of defenders adding 500+ points every 30 seconds, it's going to be much harder to completely defeat your gym.
If you're not claiming a gym yourself but trying to train up someone else's gym so you can add a Pokemon it can be a little harder, as you don't have as much control over what the first Pokemon in the draft will be. This is where it pays to have Pokemon of all different types at all different levels -- even as low as 400CP. The more variety you have in Pokemon, the more gyms you'll be able to level up quickly and painlessly.