I know the guys sometimes had love interests on the show but did any of them ever have a steady girlfriend who appeared on more than one episode?

Not that I recall. The one I always had a crush on as a kid was that blonde girl who played Toby (I think that was the name) who worked for the magazine in the Monkees A La Mode episode. She's the one I remember the most as a consistant recurring 'girl' on the show but she was never tied to just one Monkee.

I think she was Davy's date in the kidnapping episode too if I recall.

Davy had so many love interests it was hard to keep track of them all. I haven't watched any of the episodes in years. But I do remember bits & pieces of it.