Light, light, light.  Cup the front of the sander with your left hand.  Not much pressure up there, really just holding onto the head so you can control where the sander is going.  Left, right, circles, etc.  With you right hand, hold onto the hose and plug it cord, almost not touching the sander at all.  All the way in the back.  Use this hand to angle the unit up and down to ensure the head stays flat on the surface.  Try this when you are not in rotary mode at first.  It'll let ya really get the hang of it.  Then when you switch over, you'll have a much better handle on things.

Glad to see this topic, was about to ask myself.

I found myself unconsciously pressing so hard the pad almost stopped moving. Hard to beat 25 years of bad RO habits....

Thanks for all the great advice and information. My biggest challenge was in keeping the RO90 level. I'm also using the hard pad as recommended in an earlier post. What a terrific forum.