Updating SQL Database schema

I have an application that already has a few versions out there.
Often, when users request a new feature, it necessitates a few ne fields, or maybe a new table in the database.
What would you guys suggest as the best way for me to update the schemas of the databases that are already out there?
At the moment, I have a fairly big list of sql statements that I run in the setup file, one for each time I have updated the schema.
Is there a better way to do this?