I tend to pick Pokémons with cp between 1850 and 2000, since it takes much longer for those to degrade to the point where they can be taken out with only 1 or 2 attacks, which means I get an extra combat round to notice the flashes around a gym and Golden Razz them. They also take less potions to heal, so I have more potions left for my attackers (and full revives whenever I can get my hands on those). After about 8 hours, these Pokémon are weak enough to be taken out with a single attack though, which is fine by me, since that means I can get my coins.
If there’s already a defender for my team in the gym, I also try to add a pokemon that counters its counters. Easiest example, since it’s very straight forward:
If the first defender puts a Flareon (weak to Water, Ground, Rock), then I’ll put an Exeggutor or Venusaur next to it (super effective against Water, Ground, Rock). If any attacker just picks the Recommended pokémon, he or she will probably start with one of those 3 types. And since most people are too lazy to switch attacker mid-fight, there’s a good chance for me to win a defending battle, and for them to need a revive.