I am a Canadian citizen who was here in the US on a
TN visa when I met the person who is now my wife.
We had it a little easy because we knew really quickly that we had found the right person. When I re-entered the US on my new
TN visa after the old one expired, I had just met her a week or two earlier and was certainly not intending to be here permanently. (After 2 dates, of course you don't think that!) Before the year was gone, our romance had bloomed, and we got legally married in a civil ceremony. A number of months later, we had a big family ceremony and reception. As soon as we were married, I could apply for the green card. The application itself gave me an automatic extension to my current status until such time as they could determine whether my green card application would be accepted. Eventually, it was accepted, and I got a temporary work authorization and could continue working for my employer until the process was complete. I didn't even have to leave the States!
Now the problem for you may be that she cannot renew her TN visa knowing that she is going to get married, so that is "dual-intent". I didn't know at the time that I got my visa that I was going to be married later that year, so I didn't have "dual intent."
So there are a few things you can do:
1. When the visa expires, she goes back to Canada and you apply for a "fiance visa" for her. When that comes in, she can re-enter the States. This may take a few weeks, however, and if she wants to keep her job that may not be a good thing.
2. Get married right now in a quick civil ceremony like we did and just apply for the green card right away. Heck, you can do like friends of mine did -- fly to Vegas and get married in the cheesiest Elvis chapel they could find. Then, plan the real, bank-account-draining wedding with the parents and flowers and overpriced cake and everything for a later time. The second wedding won't be the legal one of course, but if you're religious you can make it the marriage before God. Otherwise if you're not, just make it a big ceremony and a big ole fun party!
Also, contact an immigration lawyer to help you with the green card paperwork. They will guarantee is it correct, which means it will not be rejected and she will not have to leave the country. It cost us about $1800 total for the legal advice and the paperwork and the applications fees to the US govt.
Good luck, and happy valentine's day!