Keep the weight low and the boat is more stable - period
One of my favorite websites for testosterone-infused information, "The Art of Manliness" posted this link this morning describing what canoeing instructors call the "nose over..." rule for remaining upright in canoe. This is an important skill to learn as falling overboard when you're the supposed leader of the canoe trip, really damages your reputation (not to mention making you the target of interminable bad jokes for the rest of the trip not to mention an unfortunate hit video on Youtube given the number of likely cell phones present).
The classic "knees above gunwale" position
The boys in the picture to the left (Noah, Eric and Kirk) are doing it wrong. I used this photo to show what I mean about high knees. Fortunately, they were doing it right out in the deep water, but got loose in their technique once they were in just a foot of water. Also, Eric sitting flat on the bottom of the canoe helped stability immensely. Also note they are perpendicular to the Earth if you draw a vertical line from nose to navel to that special spot between the knees. If you can figure out the "nose over..." rule and learn to religiously keep the weight low in your boat, a canoe is quite stable.
As Darren Bush, the author of the "Art of Manliness" piece points out, "There are no tippy canoes, only tippy paddlers. It's really worth spending a few minutes reading this article.