I did some research for my partner as he is seriously considering becoming a citizen. My understanding is it works as follows:
You need to be physically present in Canada for 1095 days of the prior
4 years before you can apply for citizenship.
This means that within a 4 year perioud you can only be out of Canada for a total of 1 year.
If you are outside of Canada (it doesn't matter if you are with your spouse) then it does NOT count towards your citizenship (this is different from residency where if you leave Canada with your spouse it continues to count towards your residency requirement). I don't believe the renewing you PR starts the clock over again, as long as you meet the residency requirements.
If you leave and come back the same day it does NOT count as an absence. Also, when you leave, only one of your days in transit count as an absence (aka the day you leave counts as an absence but the day you return counts as being present).
For proof, keep all tickets/itineraries, if you are lucky you will get stamps in your passport (you can even ask the boarder to stamp it on your way back into Canada although I'm guessing CBA has a record of when you've left the country and when you returned.
The days you live in canada before a PR are indeed counted as 1/2 days (but not sure if there is a limit or not as this doesn't apply to us so i never looked).
missmini is correct, the process can take more then a year to complete and there is a citizenship test that you need to pass.
The website http://www.cic.gc.ca/english/citizenship/become.asp has a ton of information about this and it seems rather straight forward after the application for PR