In general I think of "to speak" as being "говорить" and "to say" as being "сказать", but I know this can't be correct, since they are an aspectual pair, and thus their only semantic difference should be that between perfective and imperfective verbs (and the words "to speak" and "to say" seemingly should have translations in both the perfective and the imperfective, not just one or the other).
In most languages that I know besides English (German: sprechen vs. sagen, Spanish: hablar vs. decir) there is a clear way to differentiate between "to speak" and "to say".
But are the two words translated interchangeably into Russian, the choice between "говорить" and "сказать" coming down to aspect only?