Leaving Canada
Really not much else to do at this point. Maybe leave some cash in your Canadian bank to hold your car payments for awhile. I would suggest starting to do research on where you're going to live which is covered later, but if you can narrow down some apartments before you even come down to the States, you'll save yourself a lot of time. You may need an Immisions letter from your car company stating that it meets the USA Immisions test.
So pack your bags, and say good bye to everyone. Take one last feel at how hot and humid it is (if it's summer), or stand outside during the frigid cold, because that's the last you'll be seeing that for a long long while (if you're going to California).
Forward Your Mail!
your cable company (alternatively, close your account with them before you leave)
You may want to consider filling out a Canada Post change of address form. For a US forwarding address, this will cost CDN $60/6 months. Do this 2-3 weeks before you leave and call Canada Post to verify that your request has made it into their computer. Canada Post will not forward to a company address. I believe they will not accept a mailbox as a forwarding address as well.