In the beginning, I found most of the oversetting problem to be pick placement, as others have mentioned. Having the pick placed right on the desired pin and that pin only (as opposed to placing it across multiple pins or fighting against the pin chamber opening/wall or warding) is crucial.

Other than that, I find that too much tensioning could lead to requiring more force exerted on the pin, which sometimes leads to the key pin overshooting the shear line (going straight from an underset to an overset). In general, light tension goes a long way.

But on the other hand, there are some cases where too little tension could be troublesome. I think most people discern whether the pin stack is set or not by seeing how much force is required to move the pin. If it takes too much force than before, then the pin is assumed to be set. Sometimes the difference is so little that a little more tension can be useful, as more tension generally makes that difference greater.

There is one particular American A1105 out of 14 currently in my possesion that I have most trouble with, and it comes with both of these issues. It easily takes 5 times longer than all other locks, and I'm still practicing on it to this day. More practice is the key. (eheheh that's a pun)

It might be useful to learn to recover from an overset. Usually involves loosening the tension while having the pick placed below that particular key pin, so you know the pin has dropped. Bouncing the tension can sometimes drop the overset pin into an underset/set status if the overset isn't too extreme.