Whilst it might be a seemingly obvious observation, I have just found that the correct temperature of water and the correct concentration of salt and bicarbonate of soda is crucial to the success of irrigation.
I have been rather lax in my approach in the past (with regards to the mixture) but after getting a bit more serious about this I have noticed an improvement.
I now only use boiled water and I clean my neti pot and Neilmed bottle properly. I'm using the pre-mixed sachets at the moment but either way I thought I would share this with everyone.