i discovered a decent technique for bowling slower balls today, bowled loads to my dad, he just couldnt pick them :P. What i do is i have only my index finger (one to the right of my thumb incase that isnt index finger :P) on the seam and then when going through my action just rub my finger across the seam and it applies spin to the ball which really slows it down. Try it out.
I've never been tested on a speed gun but my dad reckons i am bowling about mid 70's and i am pretty accurate aswell, which is awesome. I'm 17 btw and about 6ft.