Definitely try the sinus rinse. I would recommend the Neil Med Sinus Rinse over the neti pot. We have used both. With the neti pot, it helped, but was basically a slower pour. With the sinus rinse, you squeeze the bottle and I think it squirts up higher and cleans out more. My 8 year old manages it herself twice a day, and I do think it helps a lot. It is a quick, easy, non drug intervention that will definitely get results. View a quick video about it with your daughter to make sure it goes smoothly. It is a little weird sensation until you get used to it.
http://www.neilmed.com/usa/adword_sr.php?gclid=COK93LrFjK...
Also, our pediatrician told us that zyrtec was more effective than claritin for many people, so perhaps experimenting with a different antihistamine may show some benefit.
My daughter's allergist thought irritants may bother her as much or more than allergens. There is no test for these, but if your daughter is particularly senstive to smells (cleaning products, scented candles, detergents), you may want to look for other alternatives. My daughter was prescribed a liquid medicine called Dallergy, which is supposed to be better for irritant induced congestion than either claritin or zyrtec. However, I can't say we've personally noticed a difference.
One other medical intervention that may help is a prescription nasal spray (Veramyst, fluticasone, etc.)
I definitely recommend a consult with an ENT. There may be a structural issue that prevents proper drainage, and so everything backs up. They may want to do a CT scan of her head. Read up on the benefits and risks and make sure they satisfy any concerns you have about the level of radiation, because they are subjecting a growing brain to radiation. The imaging may potentially identify any structural problems, exceptionally large adenoids, etc.
Good luck to you and your daughter. It is so sad to see them miserable and not feeling well all of the time.