I recall hearing that if you forgot to make an Eruv Tavshilin there is a way to make it on the first day of Yom Tov. Does anyone know how this is done?
Shulchan Aruch (Orach Chaim 527:22) says that you should make the following declaration when making an Eruv on the first day of Yom Tov:
"If today is a Yom Tov, then I don't need an Eruv [because tomorrow is really a weekday, and I can prepare for Shabbos as usual]. But if today is a weekday, then with this Eruv I shall be allowed to bake, cook, ...."
Note the following caveats:
This doesn't work for Rosh Hashanah, because its two days are halachically considered "one long day."
According to one opinion cited in Shulchan Aruch there, this works only if you have some food already prepared from before Yom Tov. (Though Mishnah Berurah, ibid. :75, says that in practice we don't follow this opinion.)
There are variant opinions as to whether to recite the blessing when making an Eruv with this condition (Mishnah Berurah ibid. :74).
Hirhurim just put up a post discussing both the first day solution and the rely-on-the-Rabbi solution.
This doesn't directly answer the question posed, but something to bear in mind is that one who lives in a community where the rabbi makes an eruv tavshilin including "ולכל בני ישראל הדרים בעיר הזאת" can rely on said rabbi's eruv if he forgot to make one himself — but not twice in a row (viz, not two occasions in a row for which he should have made an eruv: these will often not be consecutive holidays). (No source handy, sorry.)