In reply to Smelly Fox:
Don't know about using an oven...sounds dodgy, though.
(Disclaimer...certainly no expert!) I've had a lot of success using a heat gun and digital laser thermometer off Amazon for 30-odd pounds...the critical temperature I use is 160°C. I heated the plastic slowly inside (if possible) and out...this is important (a production manager in a skiboot factory told me). Inside and outside makes sense, otherwise you're setting up some sort of distortion in the plastic I guess. Why it's better to do it slowly I don't know...same sort of reason I guess. I heated to 170°C once and the plastic starts to bubble, but certainly in my experience that's just proved to be cosmetic and hasn't led to anything catastrophic happening. I fashioned a "punch" by using bits of 1" x 1" wooden cubes, drilled to take a length of threaded bar in the middle that I can adjust with a nut. (If you manage to come up with a better solution do please share!) If you want, I could stick a photo on my album, I guess.
I play around with the "punch" until I get the right length, then heat the plastic. As soon as it hits 160° I stick on a glove and ram in the punch, then hold it in place and wait for the temperature to drop below around 50°C (again, slowly).
Also, my boots are Pebax/Grilamid, which apparently has more "memory" than standard PU, meaning it tends to want to return to its original shape. (Plus I think the same thing happens to the liners) This I can confirm because it does seem that every year I have to redo them, but now I've got my routine down, it's literally a 30 minute job.
I combine this with thermoforming my liners by putting them in an enclosed space (I used a box) and heating them thoroughly inside and out with a bog standard hairdryer. Before doing this I prepare my feet by sticking a foam patch onto the pressure point I'm trying to sort out. I've been lucky because my problem has been my ankle bones and a bit of width around my little toes, which with my boots I can access easily to get a good "punch" from the inside.
Before doing this my boots were basically unusable. Now I can ski tour all day with nary a twinge.