same tecnique I used to teach her "come" (standing in front of her, with the treat in view and asked her to come and if she came to me she got it, even if it was just a step. I take it that would be a good way to teach "forward" as well?)
I HAVE found that if we're inside that she won't really understand that she should move forward, outside she's terrific. Perhaps that might help too?
I wouldn't teach forward as in forward towards you and a treat. That would be a bribe. You want to teach the voice commands that you will use while driving.
Lunging is a great way to start (see above post by Reignmaker).
You could start by leading and using commands (walk, whoa, trot), then transfering to lunging. (You can start by going around with her, then easing further away from her until you are in the centre.)
Also, when inside you could take her to one side of the arena and aim to go back towards the door. That would add some impulsion to go forward. Or put a couple of treats in a bucket, go across the arena and head her toward the bucket of treats.
Horse training is the most creative thing I know! Have fun with it. What I've been having success with lately is working on something for 7 sessions in a row. Currently I'm on day 3 of harnessing/hitching Spinner (my paint). I suspect by day 7 I'll be standing on the sled while he pulls it. (OK, I'll admit he is a super-trainable horse with great confidence about learning.)
We also used the 7-day concept for a mini who wouldn't give his feet to be picked up (had to be drugged for the farrier and even then wouldn't give his right hind). First day we could only pick up one front foot. By day 7 we could pick up and pick out all 4 feet, and rasp 3 feet. A couple of days later, we got all 4 feet rasped.
I think the 7 day concept is great because
a) you know what you want to teach (in your case, going forward on command)
b) you start small (because you know you don't have to get it all in one session)
c) you develop a pattern of building from the day before
d) at 7 days, you can look at what you accomplished, then pick a new thing to work on for 7 days.
e) horses are forgiving, so if you screw up one day, you know you can do better next time
f) consistency!!!
Nate Bowers has a new DVD out on the foundation of driving. Google Nate Bowers.
I had the privilege of attending several days of a clinic on starting horses towards driving and it was an eye-opener. Awesome, awesome, AWESOME. I've ordered his video but haven't got it yet.