Question: I have been moving annually to a different high school in a different state since the end of my freshmen year. Given that my academic status has remained stable (though my extracurriculars have declined somewhat), how will my multiple relocations affect my admission decisions?
Typically when a student changes high schools frequently because of family relocation, admission officials give credit to those, like you, who have coped well with the stresses and struggles of repeated transfer. (This is also true if your moves have not been because your family moved with you but because you changed residences due to personal problems, a parent's divorce or death or illness, etc. In such cases, you should give admission officials as much information as you are comfortable revealing. They will respect your resiliency when they see how consistent your grades have been.)
But, as you've discovered yourself, it's hard make your mark in school or community activities when you're always the new kid in town. While admission committees do understand this, try to use your application to emphasize the benefits of your uncommon situation and also highlight whatever activities have traveled with you. For instance, if you are a musician or an avid reader or you pursue a hobby independently, you could write an interesting essay on how your "portable passion" has moved to diverse locales.
When it's time to apply to colleges, also make sure that you (or your guidance counselor) inform colleges of any irregularities or confusing situations that might crop up on your transcript. For instance, if you had to drop a foreign language after a year or two because it wasn't offered at your new school--and not because you chose to--admission folks need to know about it. If you couldn't take a particular class because of you missed it at one school and then couldn't fit it in your schedule at the next, explain that as well.
Overall, however, even if your many moves left you lost in too many high school hallways and fumbling to find your favorite t-shirt or lucky socks, they shouldn't hurt you at admission-decision time and might even be a plus.