Try on as many boots as possible. Front-entry are more commonly found and are considered to offer more support. Rear-entry boots are easy to find at garage sales now because they are out of vogue. Determine what type of buckles you prefer. Wire loops or plastic straps that insert into buckles are common.
Some boots come with heat packs or Velcro straps or the ability to change the angle of the ankle.
Pick a boot that feels good on your foot. If the boot is comfortable, you're more apt to have a good day of skiing. Make sure the boot lining is stable inside the boot and that it's removable. They're easier to dry if you can remove them when you're done for the day. Alpine boots are a substantial financial commitment, so take your time and find a boot that's both comfortable and aesthetically pleasing.