How do you avoid excessive car rental fees? Dan Bagby does it by by avoiding car rental employees.

"I always use the kiosks and apps to check in," says Bagby, who works for a grocery chain in Austin. "That way, you just click 'no' a few times to refuse the additional services and insurance."

Oh, the things car rental customers do to steer clear of the little extras. And there are a lot of little extras, for everything from overpriced insurance to pricey toll transponders.

No one knows exactly how much these fees will increase the cost of your next set of wheels, but I've read enough surveys to come up with my own estimate. I call it the 50-50 rule of car rentals: Roughly half of all renters will see an unwanted fee on their final bill, and the average bill is inflated by around 50% over the quoted daily rate, a number that includes pesky taxes and airport facilities fees.

This is not news, especially to the millions of Americans traveling this summer. But this is: They're using technology to avoid the fees.

Bagby's strategy is clever because it leverages a system that's meant to make the check-in process go faster, shortcutting a lengthy sales process. Front-line car rental agents are trained to sell you "optional" extras like added insurance; they may even be evaluated based on how much they sell, especially expensive insurance.

Sure, the terminals are trying to upsell you, too, but Bagby knows his way around the displays, and he also knows what he needs. Talk about using their own technology against them.

Or you can do what Mo Lotfi did when he found a surprise refueling fee on his rental car recently. He'd returned the vehicle with slightly less than a full tank, and his car rental agency took that as a license to charge him for an entire full tank. Instead of complaining, he got even.

Lotfi, who works for a social networking site in Hamburg, Germany, created an app called Gas Break (itunes.apple.com/app/id1093284888) that tells you exactly how much fuel you need before the car is full. When you top off a tank, you're actually filling a part of the tank that is not shown on the gauge. After just a few seconds of computing basic information about your rental car, Gas Break lets you to see the exact amount of real tank capacity. This allows you fill the tank to the line — no more, no less. And by telling the app the current gauge level of your rental car, it can clue you in to how much gas you need for a full tank.

"In our tests, we were able to save an estimated 10% in total gas price by using this app," Lotfi says.

Another source of unwelcome fees: parking tickets. They're particularly vexing to rental customers because car rental agencies add fees on top of the charge when they send them to you in order to cover their expenses (and some renters claim, turn a little profit).

Enter Joshua Browder, a Stanford University freshman who created the site DoNotPay (donotpay.co.uk) — billed as the word's first "bot" lawyer that helps you challenge traffic tickets. The site offers a series of automated responses to questions about a parking ticket, which can help you fix it on your own. He's reportedly helped motorists in New York contest more than 9,000 tickets and all told has saved drivers more than $3 million. There's no telling how much he's cost car rental companies, but they can't be too happy about it.

"From my experience, when the local government knows someone is using a rental car, they are particularly prone to issuing an unfair ticket,"  Browder says.

And then there's insurance. Upselling clueless car rental customers on insurance accounts for close to $1 in every $10 collected by your car rental company, so they are understandably protective about it. The workaround: You can make sure you're covered before you show up at the rental counter, through either your auto policy or your credit card. Or you can book through one of the new car-rental apps like Carla (itunes.apple.com/us/app/carla-rent-car-by-comparing/id1035631234), which includes insurance in the price of its cars.

Car rental companies aren't taking this threat lightly. Just last week, I heard from a rental customer in Orlando who couldn't show proof of insurance. Although car rental insurance isn't required, the rental agency still refused to rent to her. They haven't come up with an app for that yet, but it's just a matter of time.

More tips to save on your rental

• How to avoid tolls: Car rental companies add extra fees for using their transponders, sometimes charging by the day (whether you use a toll road or not). Either bring your own toll transponder or avoid tolls with a reliable mapping app. "I would suggest using Google Maps," says Nenad Cuk, a frequent car renter based in Salt Lake City. "The application has advanced settings that allow you to avoid routes with tolls."

• How to avoid high insurance rates: Car rental insurance can be found in unexpected places, including your own credit card, travel insurance policy or as a standalone product from your online travel agency. (Some are limited, so be sure to read the restrictions.) If you are starting to plan for your trip or find yourself at the counter without a collision-damage waiver policy, don't worry; you can still buy it from a site like Insuremyrentalcar.com for rates that start at about $5 a day.

• How to avoid tickets: Download an app like Speed Cameras & Traffic by Sygic (iOS, $4.99) (itunes.apple.com/us/app/speed-cameras-traffic-by-sygic/id766632719), which lets you see the speed limit for the road you are traveling on, or CamSam Plus (Android, $.99) (play.google.com/store/apps/details?id=com.camsam&hl=en), which alerts you to speed cameras. Many GPS navigation systems also come equipped with traffic enforcement warnings. Better yet, obey posted laws and speed limits.

Christopher Elliott is a consumer advocate and editor at large for National Geographic Traveler. Contact him at chris@elliott.org or visit elliott.org.