Make sure you have a signed Canadian ownership and a bill of sale. These should be faxed to your border crossing US Customs 48 hours prior to making the crossing. when the bike shows up at customs you will need to pay state sales tax.
Sometimes the border guard doesn't ask for anything, then it's free sailing. Be careful not to "obviously" understate the value; when I brought my H1B from New York to Ontario, they held me up for an hour while they scoured expired eBay listings looking for values -- and I was being honest..
