Our last car rental experience was irritating. The person at the counter who was trying to sell EVERYTHING...kept saying...YOU ARE IN NEVADA NOW. (Thank heavens...the pilot did not get lost

)
The base rental cost is one thing.
The pressure for all the "add-ons" when you pick up your car is getting obscene. All I can say in advance is - make sure you have a copy of your insurance card, and know what your insurance will cover if you get into an accident.
Also - is there a fee for additional drivers, i.e. a spouse, or a significant other?
We would never pay the fee for a GPS - we usually bring our own, or use our car phones.
Do you need car seats - it has been awhile since I've needed those., so I have no clue what current policies on those are. Check out this in advance to.
Also - make sure the car you rent will fit the luggage you will be bringing.
Taxes for car rentals are absurd (IMO.) I don't know if there is anyway around this.
It also seems like many car rentals are offering (FOR A FEE) the option to purchase the tank of gas that is in the car when you leave, and then you don't have to worry about filling the tank upon your return. I LOVE this, DH hates this. IMO - not to have to find a gas station prior to returning the car is priceless - we just watch our gague closely. Now - if we were only going to be putting on 100 miles on the car - we would never pick this option.
Good Luck!