Porting is it's own thing - you must seperate implementation dependent things and ... "business rules" fairly strictly if you are to keep it portable. I'd also

Porting is it's own thing - you must seperate implementation dependent things and ... "business rules" fairly strictly if you are to keep it portable. I'd also strongly suggest a really comprehensive test suite that beats the snot out of the implementation dependent portions. Somebody mentioned "scripting languages" - use the ability of scripting languages to construct combinators to write your tests. They migth even emit 'C' code.