If the GPA is really low (like under 3), you may have to get another degree and rock it (and incorporate all your prereqs into that new degree if you want schools to ignore your previous one). Just be sure to do it FULL-TIME (5 classes a term) - no excuse, including a full-time job, can make up for part-time status.
(Done the first, wish I'd done the second - would've meant far fewer rejection letters this year).
The old degree will be evidence that you are well-rounded, esp. if it's not in science. I suspect that post-bacc is a U.S. thing. I know there aren't any premed postbacc programs in Canada, anyway. If you're just raising your GPA by a bit, you could start another degree but maybe not have to finish it; some schools do just count your most recent X years of "undergrad studies" (in whatever program, they don't care).