If you have a friend / business associate who is also a DM book the flight together, you will (usually) automatically receive a near-instant upgrade to C+, assuming your companion has C+ upgrades enabled in their account. As was mentioned, you'll also stand a MUCH higher chance of being upgraded over GM or PM (obviously).

If I had a higher-status flying companion willing to book the flights, I'd take advantage of that every time I could. There's no way to lose, since you'll always receive full MQM / MQD and RDM credit for every flight you take.

Even if your companion has to cancel or otherwise alter their flight, you should (AFAIK) simply revert to your actual status for upgrades.