I worked for an airline fare pricing startup. The key to getting a lower fare is to not be flying for business.
Since airlines can't outright ask you if you're flying business, they'll instead offer tradeoffs that a business flier won't make. So plan in advance, but be flexible in trading off day-of-week or time-of-day.
What are some tells of a business flight? The only one that comes to my mind: I am doing whatever is possible to fly out on Monday or Friday rather than give up my weekend for the company.
I have always booked corporate flights through an internal portal system. I assumed that this identified me as a belonging to X company, so my options would be priced by some standing agreement with the airline. Is this not true?
I think a huge tell tends to be flying in on monday and flying out friday. A lot of cheaper tickets require you to spend saturday night at the destination to weed out business travel. Other people mentioned multi-stop routing and odd timing (e.g. biz travel usually won't involve leaving at 3pm on a tuestday). Some of this is obscured by day time flights being more desirable (e.g. no one wants to get up at 3am to make a 6am flight).
The other huge tell is last minute purchases. A lot of business travel ends up being decided on a week or two before the flight so last minute flights tend to cost more even though in some sense the value of the seat declines as the flight approaches. E.g. an empty seat is worth $0 and a pure loss to the airline so they should want to sell those seats before the flight even if they get minimal revenue. For biz travelers, clients often pay for travel expenses so inflated pricing for that flight tomorrow is worth it.
Crappy routing. Business travelers aren't going to fly from Salt Lake to London via LAX and EWR for example. Airlines will offer wacky routes at lower prices that obviously cost them more in terms of fuel and airframe time, because they know people who value their time (and aren't paying themselves) will not accept them. This is particularly prevalent with award travel. You can try to game their system though -- find a crazy route that you actually want to fly (12h stopover in some city you want to visit), but which the airline's algorithm sees as unappealing.
I'm not the best person to answer, but it's really any price<->X tradeoff, whether that's being able to choose at the last minute, going to your preferred airport in the destination city, or even those other bullshit upcharges airlines have.
Since airlines can't outright ask you if you're flying business, they'll instead offer tradeoffs that a business flier won't make. So plan in advance, but be flexible in trading off day-of-week or time-of-day.