If you reserve (just book, no need to pay in advance) a popular class of vehicle in advance the pick-up location will often not have one available so you get a free upgrade to whichever superior class they do have ready. Also, look for 'off-airport' locations because these do not assess an airport surcharge fee. Off-airport locations are even less likely to have popular classes in stock. Doesn't matter if you just flew in, (remove your airline luggage tags, try not to be obvious) take a taxi to the location. It will frequently be cheaper this way. Off-airport locations are often near airports.

As an American living in Europe, I found that some US insurers will issue a 'driver only' policy that covers you while driving rentals in the USA on your European license. I'm paying $24/year for mine which has generous limits and is much cheaper than the coverage rental agencies offer.

Also slightly unrelated but don't dare call British Airways to enquire about changing your flight or you will not be able to check in online and you will get the worst seats when you show up for your flight. This is true even if you make no changes, your booking is put in to some kind of frozen state and the phone agents are unable to reverse this. You must visit the BA desk at an airport in person to have this corrected. Bizarre, but this was confirmed by multiple people at BA who I spoke to after having it happen to me.