I kept the clicker in the hand that had the leash and I wear a hoody so a baggy of treats can go in my pocket. I always have a few bits of the treat in my hand though so its faster. Took a lot of fumbling but once he was tall enough it got easier .
I was told to always treat from the left hand when teaching heel. If you use your right hand, the dog will focus on that hand and you will be encouraging him to wrap in front of your body instead of staying in position.
I keep a treat bag attached to the waistband on my jeans on my right side, hold the leash in my left hand, and the extra leash and click in my right hand. When I click and treat, I try to quickly move the extra leash from my right hand to my left hand, click, and take the the treat with my right hand.