Car rental agencies want the tank full when you return the car
Make sure the tank is full before returning the car to car rental agencies. And when I say full, I mean top it off. Technically you aren't supposed to do this, but if it's on F when you leave the gas station it might be a hair under the F when you hand over the keys. Car rental agencies WILL hit you with a fuel fee if it isn't on, or over the F. Best bet is to top it off and get a receipt then go directly to the car rental agencies.
Car rental agencies want to be notified of any change of plans
Remember, they will not hesitate to charge you a 'no show' fee if you show up late for a pickup, and I mean even just 15 minutes late. Make sure you call the car rental agencies at least a day in advance if your pickup time will change. A courtesy call even if you're delayed on the way to the pickup might just be enough to avoid this fee.
Car rental agencies don't want their car back early
Make sure you drop off the car at the time you contracted for, not early, and certainly not late. Car rental agencies love to charge fees for anything, don't give them any reason to do it. Remember, they make their money when the car is rented out, not when it's in the lot so they don't want it back early. Some will charge a fee or even the full rental price if you return it early claiming they couldn't rent it because it was supposed to be out already.
Car rental agencies love to sell you their insurance
This is a big scam by some agencies. Some, less than trustworthy car rental agencies will attempt to sell you their car insurance. If you already have insurance you are NOT legally required to buy it from the agency, some will insist you must though. In fact, sometimes the credit card you use for the rental will provide some insurance. Speak to a manager or call their district office if they insist you do, you'll be surprised how quick they'll make an exception 'this time'
Car rental agencies, are they worth it?
Well of course they are, just be wary of anything they can charge you for, and of course read the contract. If you don't give the agency any reason to charge you fees then you should have a great experience from them. I should say this,just about all of the big name agencies will go out of their way to make you happy to get a repeat customer, just be aware of what you can be charged extra for and do the right thing, whether a courtesy phone call or just having gas receipts handy. The fees aren't small so you could end up saving quite a bit of money.