Keep a complete and precise log of all travel outside Canada.
For PRs without a plan to become a Canadian citizen, this information is still crucial when applying to renew the PR card (remember, burden of proving compliance with the PR Residency Obligation is on the PR).
For PRs planning to apply for citizenship, this information is absolutely necessary in making the application for citizenship. Again, burden of proof is on the person applying for citizenship.
Related to this: best to gather and retain documents evidencing place of abode (if renting, for example, copy of rental agreement plus proof of payment) and employment; be sure to file Canadian resident tax returns. Even for those who never travel outside Canada, these records may be required when applying for a new PR card or for citizenship.