As others have said, you will grow accustomed to the phen's effects. Taking a week or so off from is is an option, as is the 5-htp to boost it some. But I believe even if you take 5-htp, you will still need a week of no-phen to keep it working. It's my understanding that most drugs affecting the receptors in the brain like this do the same thing in that you get used to the dose and it stops working as well.
One thing that's cheap and helpful with phen, I have found, is water and lots of it. I'm far from a 'pro' here though.
Dumbfox, I would ask what bontril or tenuate are? Different meds that do the same thing as phen, or just different concoctions of the same thing?