My instructor will have me get in my two point before the start of each lesson for 2 or 3 laps and that helps. She also will tell me to think of lifting my toe instead of dropping my heel so that my leg doesn't swing forward.
When I was having trouble keeping my heels down, I would point my toes in as much as I could and it helped improve the flexibility in my foot which lead to keeping my heels down because it was comfortable and there wasn't any strain :)
It's good to flex your heels out of the saddle too! Stand with the ball of your foot on a stair and flex your ankle down. Other than that just consistency and practice are the main things!
I agree with Rachel, also you can go on the stairs and at the edge put ur heel down
Practice riding while at 2point(standing) then to riding without stirrups. This will force you to keep them down and the muscle memory will transfer to when you have stirrups in!