I did purchase a lifetime TWA lounge membership for $600, but it was canceled about a year or two into the AA takeover. Nevertheless, I suspect that the cost was roughly equivalent to purchasing annual lounge memberships for the applicable time period.

I wish I had taken advantage of the Orient Express train tickets offered as part of some TWA awards. I tended to pick the 50,000 RDM award for one IFC RT to Europe "free' ticket plus an upgrade to IFC TATL RT from any coach fare for me at some later date, but I never was able to use the associated free Orient Express ticket; the dates didn't work.

Not exactly a FF program perk, but I wish I had taken advantage of the offers to take Concorde TATL in one (the less popular) direction and take a cruise, IIRC on the QE2, in the other direction for roughly the cost of a business class TATL RT ticket.

Fortunately, I did once benefit from a NW offer through AmEx PTS to ride in IFC for the price of business class TATL to/from Japan, of course on the 747. This when there were Japanese FAs who changed into kimono to serve tea and the Japanese meals in first class. It was such a nice introduction to Japan.

Another thing I did right was to use PMNW/KLM RTW business class fares when I needed to visit Europe and Australia with not much time in-between, which got me a more-than-free stopover in Hong Kong shortly before the handover.

Similarly, another time I used PMNW (with QF) Circle Pacific business class fares to visit several cities in Australia and save a lot of money. Returning TPAC to the USA on QF in business class entitled me to use all of the QF domestic and international business class lounges in Australia.

During the period shortly after the DL/NW merger I used my old PMNW miles to redeem four RT IFC TPAC award tickets on SQ plus one in business class at 160,000 and 140,000 RDMs per RT respectively. It was surprisingly easy to find availability to SIN, NRT, HKG, and DPS using DL miles even though *A and even SQ award space was more limited. In fact, on the business class ticket, I had a DL domestic FC connection flight LAX-MSP and was erroneously invited to use The Private Room on that SIN departure. I also remember going to SIN the evening before a morning departure on at least one of the IFC award tickets, checking in to the transit hotel, and going to TPR for dinner and then again for a light breakfast before my flight the following morning, thanks to the SQ policy permitting check in at T-48.

I rarely fly AA/oneworld, but perhaps I should have moved credit card spend from AmEx to get AA status as a fallback while this was possible. I definitely should have pursued making sure that AA gave me full credit for my TWA lifetime miles, but after the takeover I didn't think I wanted to fly AA anyway given that most trips would have required a small plane to/from MSP. It also would have been nice to manage to get UA lifetime GS status while that was available for reasonable terms all things considered.

Still, some stuff I did right was to sign up (for free by snail mail, so with a lag of up to two months) for virtually every domestic FF program in order to avoid ever taking a flight that didn't credit somewhere while also realizing that the only way to get decent FF program benefits was to try to concentrate my flying on one or at most two airlines, TWA in my case. Back then, I also carried a hard copy of the program rules with me--TWA mailed a wallet card every year with the new elite card and bag tags every year--with me in case when I needed to show it to GAs to prove that I had an upgrade-eligible fare.

When I first started flying PMNW much, upgrades were given on a first come first served basis when one called in shortly after midnight. Since I'm naturally a night owl, this worked extremely well for me, although sometimes I would call in shortly before midnight in order to increase my odds of being connected to an agent right at midnight (in the time zone of the flight's departure).