Outdoor Power Equipment and Small Engines - Craftsman mower powerdrive

Is there an easy way to adjust the power drive handle on my craftsman mower? It's an older model that you switch into drive after starting. I looked under the cover and it looks to me like it's basically similar to a hand brake for a 10 speed bike. The problem is it slips out of drive and I end up holding it in the forward position which is getting aggravating. My last mower was a toro which I was taking to my local repair shop yearly for tune-ups etc., but it seemed as though they ended up doing more damage than good. After much frustration, I put it out front and it was gone within 10 min. I wish I knew more about these things. I know (or rather I'm hoping) it's pretty simple. Thanks for any and all info. Mary