Practices to keep SQL Server databases easy to port to MySQL?
Yeah, plan on using none of the SQL Server features like Geography data types, SQL CLR, filtered indexes, top, CTEs, etc.

I'd say don't bother trying to program for multiple database platforms.  Pick a platform and that'll be the platform which the app runs on.  If the app needs to be moved to another platform later deal with it then.  Otherwise you'll spend so much time trying to figure out if the code can be ported to both platforms that you'll get no where.

If you are going to plan for a MySQL migration, why not also plan for Oracle, Sybase, NoSQL, Progress, etc?