From the very beginning, I have trained my Beardie the leash is not a bad thing - there is no difference to him whether it is on or off. Sometimes it's dragging, sometimes I'm holding it, sometimes it's off.  If I have to call him back to re-leash, it is always followed by a game of tug, a few ball tosses or a cookie. Sometimes all three!
I never want him to learn the leash is the end of good times. It doesn't take long for a dog to learn once the leash goes on, the fun stops. 
For us, the leash is predictive of potential treats, games, sniffing and chasing!