For me it's simple:
1. DO NOT UNDERESTIMATE YOUR OWN POKEMON --- Why? Because bad trainers raise up only bad pokemon
2. As much as possible DO NOT EVADE TRAINERS --- Luckily, there are places where there are nurses
For example: Pinwheel Forest --- Outside, Resort Desert --- Near the Gate, Chargestone Cave --- 2F, Victory Road --- I'm Sure there are more.
3. If you are EXP-Go-Getters, look for Audino they're good but also consider your EV's
4. There's always the league! At your service!
5. Treat them with drinks (Fresh Water, Lemonade and the like)