In order to maximize your chances at being comfortable, make sure to do the following in this order:
1. Be sure that this is the right boot for your foot- not simply the boot that you want to be in. If this is the wrong boot for your foot shape, then chances are high that you will be uncomfortable no matter what. Did the boot-fitter select this exact boot for you given your foot's length, width, and volume?
2. Get footbeds that properly support your foot
3. Get the liners heat molded
4. Wear your boots & footbeds in the house before you go skiing (20-30 minutes a day, a few days before your ski day)
5. Stay out of the backseat. You could be in the perfect boot but if your technique is horrible, then you will be skiing in discomfort.
If you follow these steps, in this order, you will have a very good chance at being comfortable in your ski boot.