I traveled heavily during the late '80s and early '90s, compliments of the US Army. Much was in military aircraft, but most was commercial.
We were allowed to join frequent-flyer programs, provided we didn't redeem the mileage for personal purposes. It all had to be for some kind of enhancement to our official travel.
So I could buy First-Class upgrades, and VIP lounge memberships, but no personal travel tickets, and no redemptions for products like luggage, or other items. I always bought the VIP lounge passes, and First-Class upgrades, so that I hardly ever flew coach, unless I was in uniform, which is prohibited in First Class by Army regulation.
I enhanced my mileage points by taking car rentals with participating companies, and likewise staying in hotels that earned me mileage. I could get 10,000 points on a single trip, and I routinely earned 6-figure points annually.
When I retired I had several hundred thousand points accumulated with Delta alone, a member of their highest frequent flyer club. Since I wasn't on active duty anymore, I finally redeemed them all for private use, since I doubted the Pentagon could track it and prevent me at that point.
The trick is to use every participating program the airline offers, from rental cars to hotels to restaurants to merchants and credit cards. And watch for expiration dates, which weren't a big factor when I flew, but are tighter today.