I think saline nose rinsing can help with several problems with the nose and sinus-cavities. It can help a lot but it is no silver bullet and there are also situations that it should not be used and other cases where it might not work.
My idea on the best rinsing solution:
For long term daily use or if the nose is irritated or infected I think it is important to use a rinsing solution that is the least irritating.
The best I know is an isotonic saline solution buffered with bicarbonate (Natriumhydrogencarbonate or NaHCO3) to obtain the optimal osmotic value and PH (acidity). An equivalent of 7,14 grams of table salt (NaCl) and 2,69 grams Natriumhydrogencarbonate (NaHCO3) in a litre of water should give this ideal solution. Without bicarbonate just over 9 grams of table salt should be used in a litre of water. After a short time of use many people can taste if it is the right salinity. The temperature is also important and should be comfortable to the nose so use lukewarm water of the right temperature (feel it with your finger or mouth to test it).
The easiest way to make the solution with the right concentration of salt is to buy a neti-pot with a measuring spoon.
Other rinsing solutions:
Using too little salt will damage the nose. You can also use too much salt. Never use less salt as the isotonic solution described above. The solution can be made up to a maximum of three times as salty but that is the maximum. Increasing the salinity much above a isotonic solution might irritate the nose and not be the right thing for frequent use.
Occasional use of a more than isotonic saline solutions can be good for freediving because it enhances the decongesting effect of the nasal wash, but be careful not to irritate the nose which will give the opposite effect. A tiny bit more than isotonic solution is my preferred solution and before a competition up to 2 or 3 times the amount of salt for a isotone solution can be used in case one has a bit stuffed nose.
Kinds of salt:
Several kinds of salt are used: sea salt, mountain salt, special Himalaya salt, table salt. It all works. The osmotic value per volume is slightly different because of the different fine-ness of the salt and also the osmotic value per gram of salt might differ slightly for every kind so the amount of salt used should be adjusted to that to avoid to take to little salt. In case of doubt: use a little more than the isotonic solution. A lot of salt contains anti-clogging substances so the salt can take a little moisture without clumping together. Those anti-clogging substances may be acceptable for the stomach but might irritate the nose you are trying heal. I therefore would go for pure salts without anti- clogging substances, which can be bought at a pharmacy and some supermarkets.
How often:
A lifetime of daily use is possible and as far as I know for up to a few weeks more than once a day is possible as well. It takes a few days of regular use to reach the optimum effect.
How to do it:
I think that gently letting the saline solution rinse through the nose is the best way to do it. In in one nostril and letting it flow out of the other using plenty of saline solution. A neti-pot can be very helpful. I do not like to forcefully inhale or blow out the water.
Neti-pot:
A neti pot can be helpful, especially with a measuring spoon for the right amount of salt. Buy a cheap, easy to clean plastic one to try it. They should be around for 20 euros or less including a measuring spoon and shipping to your home.
Just my thoughts, but maybe it helps anyone.
http://www.apnea.nl/Medmed.htm
http://www.nasespuelen.de/englisch/index.php
www.yogaskolen.org/english
www.tarayoga.net/neti
www.hydromedonline.com