We have two ways of doing it currently.
Old and busted.. we have a series of blank drawings that only hold style definitions.
ex.. 1 drawing for wall styles - 1 drawing for window styles - 1 drawing for display configurations. - 1 drawing for dimstyles etc.. Then we run a script to purge out the existing styles, and bring in our standard styles by inserting all these drawings every time a drawing is opened. All definitions ready and waiting for you. Goes faster than you may assume too.
Why the separate files? We have 6 different regional offices, and we want to have as close to one uniform standard as possible, while still leaving room for one regional office to have their own walls, or doors, etc. That's why we have them in separate files. If Florida wants their own wall styles, then their office gets a Florida specific version of the wall styles file loaded to their server for our update script to make available on everyone's system in that office.
What we're moving to is similar file structure wise, but we want to start utilizing tool palettes to bring in the styles and definitions in a more "on the fly" approach. This would eliminate inserting the drawings every time one is opened.