For me it's not important if I sit or lie down. However I seem to go deeper, when I lie down. But that doesn't matter, because you can accomplish lots of things in a light trance.
It's hard to tell whether you are in a light trance or not. There are no signs. But chances are that just by sitting or lying down nice and quietly and relaxing totally, you WILL be in a light trance. In a deeper trance, I get the sensation of drifting off and not hearing some of the suggestions that I have put into the file, and then suddenly "waking" and becoming more aware. - and then drifting off again. It happens all the time. That doesn't mean, that what you can't recall hasn't been heard by your subconscious. You can be sure, that it has - as long as you don't fall asleep instead of going into a trance!
But one thing is important for me, when I want to go into a trance: I need to feel absolutely sure, that I can do it without being interrupted. If I'm not secure in this manner, I can just forget about relaxing enough to go into trance.
Regards
Carsten