It's pretty simple, really....
DO'S
- Make your character believable (meaning no Mary Sues/Marty Stus) People HATE those know-it-alls without flaws
- Continuity is a must!
DONT'S
- DO NOT, and I stress, DO NOT Make it the predictiable fic.
IE:
Trainer X gets powerful Pokemon X from some old geezer of a professer named after a tree, and Trainer X wants to go into the local League which is called The (insert color of your choice here) League.
PEOPLE. HATE. THAT.
- Don't give your trainr an absurdly powerful starter Pokemon. I've seen fics with the star getting an Aerodactyl or Mewtwo right off the bat. Beleivable? No. Sucky? Hell yes.