I use one of those micro joysticks with an index finger with my palm resting on a hard flat shaped pad, and yes it is very difficult to keep it just in the right place when going over any bumps in the roadway. It is frustrating.
To fix this problem correctly, one day I plan to put some really deep grooves into my pad so my hand sits down into it and really can’t move once it is there, but I haven’t taken the time or effort to figure out how to do that. So, in the meantime when I am out and about I put a 4″ wide strap over my entire hand and fasten it down on either side of my pad using Velcro. If I have it tight enough, my hand is prevented from sliding side to side and does a much better job of keeping my finger in the right place. Unfortunately, it can still slide forward and back with some terribly aggressive bumps or hills I go over, but it’s better than nothing! It also keeps the random stranger from trying to shake my right hand since it isn’t easily accessible for them to find!