To me, a pull off is little more than a string pluck. If you can pluck the string sith your 'strumming' hand, you can do the same with the fretting hand. Doing the hammer/pull off deal is the hammer with a little downward pull until the string wants to release for the pull off.

I know, easier said than done.

However, if you worry less about striking the lower string, you'll probably be better. Reason being, you can't not touch the string below when doing a pull off. So, work with that action. I noticed that when I do a hammer/pull off deal that my finger immediately touches the string below but in a way that is muting/deadening and does not resonate the string.

If you try to not touch the string, you're going to kill yourself.