Make sure you have the correct amount withheld from US income received. Generally amounts withheld in excess of treaty rates will not be creditable in Canada. In order to get the a refund from the IRS, you will need to file a U.S. 1040NR return and apply for an ITIN (individual taxpayer identification number) with the ITIN office.
Waiver forms such as the W8BEN should be submitted to the payor prior to the anticipated receipt of any US income to ensure the lower treaty rate (which could be 0%,5%, 10% or 15%) in lieu of the US IRS code withholding rate of 30%. Interest, dividends, royalties, pension are usually the types of income that are overlooked.