The results were noticable immediately.  Very rarely was I waking up congested anymore, and the combination of that change and the neti pot have been keeping my sinuses feeling mostly clear for months now.

practising daily neti really helped the sinus problems i had a few years ago, but i think the originol problem was  that gluten in my diet was causing the over production of mucus.

As for the sleep thing, since you're getting different blockages then yeah it might be different for you; it's possible that the overall best approach is to change it up from time to time (sometimes with your head mostly flat, sometimes propped up), so all the sinuses get some relief.  The big leap for me was just realizing that my sleeping habits might be relevant, and in retrospect they definitely were, so give some changes a try and see what happens.

TP

ETA: Kinsei, I've been thinking about this sleep question and I think my trouble may be that I'm sleep propped up, the opposite of yours.   The sinuses that get blocked for me of the maxillary sinuses, the lower ones and their openings are at the top so they drawn upwards in defiance of gravity.   So it seem if I sleep still with my head slightly upward they never get a break....but then all the upper sinuses drain downwards, hmmm.   I'm going to buy that Neti pot additive it looks promising.   There were two kinds one with zinc and one without, any recommendations?

However, the number one thing that made all the difference in the world in warding off sinus problems and ambient congestion in general, is changing how I slept.  I'd traditionally been a stomach-to-side sleeper (start off on stomach, find myself gradually turning to the side in the middle of the night), and so I only used one pillow to sleep on, with my head being relatively flat (so it didn't hurt my neck when I'd try to sleep on my stomach).

But then after visiting an E.N.T. and looking at some CT scans and really understanding how the sinuses are structured, I realized that the way I was sleeping was preventing my sinuses from draining at night (my head was mostly flat, so a lot of gunk would just get stuck in there).  So as an experiment, I decided to add another pillow (to raise my head more) and start off sleeping on my side from the getgo, so as to put less direct pressure on my face.

The results were noticable immediately.  Very rarely was I waking up congested anymore, and the combination of that change and the neti pot have been keeping my sinuses feeling mostly clear for months now.

Anyway, think about how you sleep and see if this may be a factor for you too.