There are no cheat codes, Action Replay codes, or built-in cheat menus for Nintendogs (DS) or Nintendogs + Cats (3DS).
Tools: PKHeX (supports Nintendogs saves), Nintendogs Save Editor (GitHub)
Best Advice: Play 10 mins/day. The game is designed for real-time bonding. Cheating deletes the only content: the relationship.