which reminds me ...; come to think of it ...; now that you mention it ...; on that subject ...; so, if you say ...
そういや、明日の予定を確認してなかった。
Which reminds me, I forgot to check tomorrow's plans.
そういや、先週も同じ話を聞いたな。
Come to think of it, I heard the same story last week too.
そういや以前、彼は似たような経験をしたと言っていた。
Come to think of it, he used to say he had had a similar experience before.