* Most US car rental price comparison sites let you book a car months in advance to lock in the price, but you don't pay till pick up. There's almost never a penalty for booking and not picking up, so I used to just block book the weekends months in advance, whether I needed the car or not.

* I just used the big rental firms - at the time services like ZipCar were priced to heavily incentivise you to only drive between ZipCar areas and drop off, e.g. use a different car for the outbound journey and the return one. Using them for all-day tripping often ended up being significantly more expensive than a traditional rental. I've no idea if this has improved. Car sharing services appeared to really rely on both your start and end destinations being in their 'coverage' areas though.

* Once I got to know staff at my local rental place, I started getting 'free' upgrades a lot.

However, after four or five months I ended up buying a modest low mileage 3 year old Honda. The cost of running isn't actually all that much more, and now I don't have to factor in travel time to rental place, having to refill the rental before return, clock-watching etc. This is entirely selfish of me of course, but very much caused by the small 'barrier' you mention.