I don't think there is any cheats except the competition cheat. If you don't know how to do it here is how.
1. Set the time to like maybe 1 minute before midnight (if you can't understand that it's 11:59 pm).
2. Go to Nintendogs again and wait the remaining time with the game on.
3. When it is midnight on the clock try doing a competition.
The Result:
Your dogs should be able to do another 3 competitions (thats if they used them up already).
If it doesn't work it is understandable as I have never tested this out.
Yeah, um, I know that one, but thanks for trying!
Anyone else??