Yea, its a nightmare to change from being a heel striker to a mid/front foot lander. When I sprint, it is always mid/frontfoot. But anything slower than 400m pace has historically had me heel striking.
I was told to work on this after developing achilles tendonitis over a year ago (apparently heel striking flicks the achilles and can cause damage over time).
You'll almost never see a world class runner heel strike. It is inefficient to the runner as it effectively slows you down before you push off again. There are some great You Tube videos that show this in practice.
I've found that purchasing some minimalist trainers has really helped. The heels in most running shoes are so damn thick that it is almost impossible not to heel strike.....and you don't feel discomfort when you do because of all the adiprene/DNA/Gel/Air bollox that is in them.
But if you get a pair of Adidas Adizero pros, believe me......you won't want to heel strike. They really punish poor technique. That said you shouldn't wear them everyday for every run. But they will, with the help of some You Tube instruction, get you started on the right path to mid foot landing.