Ive found it easiest on the couch with him sitting down i can hold onto his shoulders. he can hold my hips and help me go up and down. Also you and thrust your hips back and forth and it will give him the thrusting he is looking for and it you can still grind for clitoral stimulation
When I top on the bed, I put my knees next to his body, I can't be on my feet due to my knees being shot but I simply rock and bounce, using the mattress to help with the bouncing, he'll help by thrusting up.
When he's on the couch, I let him move me by my hips, making it easier on my knees. I mainly bounce and grind when we're on the couch.