Another substitution is mashed cauliflower for mashed potatoes. really drops the carbs. If calories are not the issue you can top it with butter and sour cream or no-fat plain greek yogurt (tastes a lot like sour cream).
There are a lots of low-carb recipe books these days, given the popularity of the Atkins and South Beach diets. I've found some good ones in "500 low carb recipes"
And if you really want to go all out, you can try Bernstein's diet - only 30 g or so of carb a day. http://www.diabetes-book.com/
I haven't tried that yet.
But as others have said, lowering the amount of carb you eat dramatically reduces BG swings and can reduce the amount of insulin you need.