Summer 1980. Looking at graduate schools.
MBS-DTW-IAH-AUS & AUS-IAH-DTW-MBS. Bought 3 legs each way because it was alot cheaper. About $275 RT. A mix bag of airlines.
Airlines: Republic Airlines, Texas International, CO, NW (orient?), & SWA. I like CO the best at the time. Got meals on most of the legs. Southwest had the cutest FA's IIRC. ^
Joined several frequent programs in 1981-82 whenever I bought the ticket on the respective airline. Didn't cost anything and why not? All the flying I done on CO in the early/mid 1980's in grad school later got me a free Australia ticket.

Included in that were a couple $16(each way) IAH-JAX tickets on CO in the early 80's. No internet back them, just spent some time on the phone securing the tickets.