Database schema version control techniques

I am in a development environment which is prototyping a database application. Currently we have 4 beta sites with different executables and database schemas. As the application evolves there are additions/modifications to the schemas. We have taken a rolling release approach to the executables, but will soon strike a version. What aides are there to maintain some version control on the database schemas as this application develops?? It seems that there is some tools/tricks available. All responses appreciated Steve Wallick Harris Publishing spw@sur1a.ess.harris.com