i haven't had a relaxer since i started wearing my phonies and half-wigs, but i imagine that when i do (aiming for 16 weeks - i may go longer now that i'm comfortable with half-wigs), i'll follow it up with a nice protein treatment and deep conditioning the same day..... i co-wash 4-5 days a week, so i stay pretty moisturized.... and at least once during the stretch, i do a henna treatment.....
i don't wear anything under my half-wigs... i tend to put it in a low ponytail and twist it up... my hair stays moisturized all day (it doesn't fully dry until i take it down when i get back home later that night - so that's like 12 hours of moisture a day)....
one thing i have to get back into the habit of doing is making sure i'm protecting my hair at night... i've gotten lax about that and i'd hate to have a setback due to it...
ETA: i'm 11 weeks post