I believe the key to successful use of non-punitive discipline methods, whether you call it "gentle" or "grace-based" or "positive," is to have a tight action point. It's consistently following through on words to "make it happen." (See also, Get Off Your Butt Parenting).
In a sense, a tight action point is what allows one as a parent to never punish and yet not be permissive. It's the "firm" in the "kind and firm" harmony that Jane Nelsen's Positive Discipline emphasizes.
As Miller and Turansky say,
An action point is the point in the interaction where you stop talking and start acting by giving a consequence.
For those of us raised in a punitive way, "consequence" may immediately conjure ideas of punishment. But in this case, the consequence simply needs to be a move by the parent to help the child comply with the command.
One way to establish a tight action point is to use the Five Steps outlined by Crystal Lutton at Arms of Love Family Fellowship.
As Crystal says,
Help is not a punishment, it is help .... It is an acknowledgment that your child is unable to stop on their own. This may be due to a lack of maturity, being tired or hungry, or simply not wanting to stop.
For very young children, the steps can (and probably should) be condensed.
I use it like this:
"Pookaloo, it's time to go. Do you want to climb in your seat yourself or do you need help?" If she doesn't begin climbing in her seat, I then say, "It looks like you need help," and then put her in the car seat. She may have a big emotional reaction to this, with which I will empathize. "You wanted to climb in your seat by yourself. You can try again next time," or perhaps, "You were having fun and didn't want to leave. We will come back another time."