Hi BossyWife - I am vulnerable to sinus infections too, though I haven't had one for a while. I believe sinus infections are common in people like us with low white cells and MDS.
I've found Nyal Sinus Relief tablets (see
http://nyal.com.au/product/nyal-sinu...t-tablets-24s/), which are herbal, to be very helpful. Hopefully you can get them in Canada. I start taking these as soon as I get symptoms such as a post-nasal drip or blocked nose. Another hint - avoid dairy products, especially those which contain sugar. This combination can produce lots of mucous and congestion in some people and thus provide the perfect environment for bacteria to flourish.
Finally, as Faye has posted, if you can get rid of the mucous it helps a real lot. A Neti pot and salt water mixture can really help with this - see
http://www.wikihow.com/Use-a-Neti-Pot