First off don't inflate it all the way! Most say that about half way is good. It's not like you're using it for padding, just warmth so you don't need it inflated all the way.

The other trick some people have done is the same as for keeping them in place on the tent floor - put small dots of McNet seam sealer on either the pad or the hammock.

But mostly, there's no good way to keep a pad in place in a single layer hammock like the HH.

You could go do some research over at hammockforums.net but the sliding mattress in a hammock is why the majority use underquilts instead of an inflated pad.

Use a bigger mattress. I use a 25 x 77 Thermarest Prolite. I never have any issues sliding off.  I have a double layer hammock which I initially bought to slide the pad in to. But, I find that fiddly and unnecessary. I just sleep directly on the pad with a quilt over me.

In colder weather I have a Thermarest sleeping bag which attaches to the Thermarest pad kind like ShawnD describes. Winter camping I have added an additional reflective pad to this setup which slides in between the layers for added insulation.