Also read this if you're considering land border entry: (Dpenbill explains the POE processes in greater detail in his post, this is a condensed version.)
Practical stuff surrounding a land border entry:
How it Works:
Fly to JFK/NYC.
Rent a car from a company that will let you drop it off in Toronto. (Most major companies should allow this.)
Drive to Toronto. (There are a couple of border crossings you could use but from what I've read most tend to use Rainbow Bridge/Niagara.)
Drop off the car in Toronto. Take a train/fly to your final destination.
If you don't drive, fly to Buffalo airport via JFK. Take a taxi to the border. Cross as a pedestrian. From the Canadian side take a taxi to the nearest train station/airport. Fly/taken a train to your final destination.
What Happens at the Border?
1 of 4 things:
1) The first officer you encounter at the booth lets you in without asking any questions.
2) The first officer directs you to Secondary Inspection. The officer at secondary inspection hears your H&C reasons and lets you into Canada.
3) The officer at Secondary doesn't agree with your H&C reasons, so he prepares a report. The report gets forwarded to a senior officer (Minister's delegate) who reads the report, asks you to explain your H&C reasons, finds in your favor, and lets you enter Canada.
4) The Minister's Delegate doesn't agree with the validity of your H&C reasons. He agrees with the report and issues you a Departure Order. (An order terminating your PR status.) You will still be allowed to enter Canada, but you will need file a notification of appeal within 30 days.
Note that in all the scenarios you can enter Canada. A Permanent Resident CANNOT be denied entry into Canada.
Which Scenarios are Good? Which are Bad?
Scenarios 1 to 3 are positive. Your PR status will remain safe as long as you don't leave Canada for 2 years. (At which point you'll be back in compliance with your residency obligation.)
Scenario 4 is not good. You can still enter Canada but you need to appeal to retain your PR status. It takes a year or two to get a hearing date. At the hearing you will either retain your PR status or loose it. If you loose the appeal you'll need to leave Canada.
Can I/Should I Travel Abroad or Renew My PR Card?
If your encounter scenarios 1-3 you must not renew your PR card for 2 years. You must not travel for 2 years. I cannot stress this enough.
Note that your PR status is not the same as your PR card. PR status is like citizenship, it's permanent and you keep it for life.
The PR card is like a passport, it's just a travel document.
A PR does not need to have a PR card. Your landing papers indicate that you have PR status. (Like a birth certificate indicating your citizenship.)
Your PR status DOES NOT expire with your PR card. You still are, and will continue to remain, a Permanent Resident of Canada, unless you are reported and loose an appeal. (If you choose to appeal.)
If you encounter scenario 4 your PR card will be renewed automatically while you wait for an appeal, but you still shouldn't travel abroad if you'd like to win the appeal. You remain a PR while waiting for your appeal and as such can live, work, and study in Canada.
What Happens Once I'm Inside Canada?
You'll be inside Canada as a Permanent Resident, and therefore are entitled to all the rights associated with your status, including the right to live, study, and work without restriction.
Do you have a SIN card? You cannot work without this.
To get essential documentation. (Driver's license and health card) your CoPR (landing papers) and foreign passport will suffice in the Province of Ontario.