Its not an exact science! But its generally advised this way...

Three Thumbs Up: Use this for one or two of your favourite shows.

Two Thumbs Up: Use this for your top ten or fifteen favourite shows.

One Thumbs Up: Use this for all programmes you like

It will take Tivo weeks to learn your habits and likes/dislikes (at least mine did) so don't expect it to record everything if you only gave it one thumb up on one occassion.

Mabe if you gave Buffy one thumbs up every day for a week it would start to record on its own.