I've found that almost every FBO will offer some discount to Angel
Flights, IF THEY KNOW YOU'RE COMING!

Just call a day or two in advance. If they don't have something
already set up, the manager or owner will almost always make
arrangements for a discount (I've never had them fail to offer). Don't
count on whoever happens to be at the desk when you get there to figure
it out - they might not feel they have the authority to discount your
fuel.

Some FBO's (e.g. Eagle Creek Aviation Services at EYE) regularly give
nice discounts to Angel Flights, but want proof that you're on a
mission (e.g. a fax from the home office). It's not a trust issue, I
believe they just need proof for their taxes.