One of the possible causes could be a fungal infection of your mucus membranes, but not that much is known about a cure for that. Google "Jens Ponikau" to see if your symptoms correspond with his findings.
It can also be an anaerobic bacterial infection, which sometimes causes smell problems with no other symptoms
A neti-pot won't reach into your upper sinuses because of gravity. The following technique uses gravity instead of fighting it.
http://www.medhelp.org/user_journals/show/2322
The Flip-Turn Sinus Flush is mildly risky, because you have to bend over to do it, preferably in a shower, but you can also do it outside on soft ground, or you can kneel down and lean over a bathtub with the shower running.
If you don't get any results after 10 days of flushing, then it's probably not an infection you can remove with saline.