Before you spend money on a custom liner or new boots, there are some simple things that you can experiment with to try to snug up the boots. They have worked for me with boots that were the right size to begin with, but were packed down so much that I could no longer get a precise enough fit by adjusting the buckles.
As was previously mentioned, you can put some shims in the boots to take up excess volume and to move your foot up into a somewhat narrower part of the shell. You can experiment with this using the cardboard that is used to package 12 packs and cases of beer in cans. This material is dense and pretty incompressible, does not absorb moisture easily, and is thin enough that you can experiment with it by building up layers until you determine what thickness works best.
I use the bottom of the boot liner as a pattern to cut out the shims which go in the shell under the liner, rather than in the liner under the footbed. Once you know how thick the shim needs to be you can replace it with something made out of sturdier material, although I've had shims made of this stuff last all season.
You didn't mention whether or not you have custom foot beds. If not, you might consider having some made for you. They will help stabilize your foot, and they may also take up some of the excess volume in the boot. If you don't want to spend that much on footbeds, you could try Superfeet after market footbeds which for about $20 are a lot better than the ones that come with the boots and do take up some volume.
If your boot shell is a good fit, these fixes work surprisingly well. If the shell is too big, there really isn't any good alternative other than new boots.
Hope this helps.