Believe it or not, sealing with mineral oil. In the winter I'll sometimes mix vo5's hairdressing with some MO, even.

I thought there was NO way this would work. I fully expected to have to wash it out the same day.

To my surprise, my hair was fabulous. Very little wilt or frizz through humid days outside. It seems to hold the moisture in, like any sealer oil does, but it lasts longer and can make it through the whole day and then some! I got 2nd day hair a lot more often after using it!

In the winter it seems to hold the moisture in, and perhaps give a bit of a protective barrier from the cold.

Another tip I've found for the winter is use a humidifier beside your bed in the winter (or if you deal with low dews) when you want 2nd day hair.
2c/3a/3b, Fine/med, Dense, high porous, & normal elas.

Lo-poo - Renpure Keratin, SMR, Giovanni SaS
clarifying - mix Johnson's H2T with low poo
Co-wash - Suave Nats. clar. or coco.
RO - GVPCB, Ion Eff. Care, TN.
LI - SMR, Cure Care, or RO
Seal - MO or Jojoba
DT - + honey, ACV, & EVOO to SM masque & GVPCB
PT - Gelatin!, CNPF
Styling - FSG, AIF, BRHG, LALS/PS, GF curl spray gel, SM Souffle/Smoothie, LOOB
Oils - MO!, EVOO, jojoba, GSO, SM Elixer, EVCO, VO5 Hairdressing