You'll have passport pages stamped when you enter and leave Canada -- this should be all you need to show time spent in Canada.
If you spend considerable time abroad, and you are married to a Canadian citizen, you should know that that time counts toward your PR quota. Here are some ideas I received from Leon.
Keep bank records (ATM withdrawals from each of your debit cards on the same day, same place), receipts from credit-card purchases (on both cards, same day, same place, and keep the signed receipts to show signatures). These would be reasonable proofs that you were in Canada, or abroad with your Canadian spouse.
But when abroad, those proofs could be challenged (unreasonably, but challenged nonetheless). So, why not take a photo of yourselves (always assuming you are married to A Canadian citizen) once a month, with a newspaper showing the day's date.
I am looking for less onerous ways, and will report back if a few comemy way.