If you frequently rent cars when you travel, you already know this expense is highly variable. The price of your rental can surge astronomically from day to day, is seasonal and can also depend on the rental company and its location.

It’s best to be proactive. The best thing you can do is compare as many options as you can, check around for pricing every few weeks, and book refundable car rentals so you can cancel if you find a better deal.

Here are some of the best ways to ensure you get a good deal.

Shop around: Because prices vary so much between car rental agencies and discount vendors, one of the best steps you can take is just shopping around. Search all the popular discount sites like Expedia and Priceline for their rates, or use a service like Kayak.com to compare rates.

You can also shop for rentals on Costco Travel, which is notorious for offering some of the best rates in the business.

Book early, then re-book: Another great way to save money on car rentals is booking refundable rentals and re-booking when prices drop.

Earlier this month I booked a four-day rental in Switzerland for $380. After keeping an eye on prices, however, I re-booked the same car with the same rental agency through Expedia for just $204.

Use Autoslash: Autoslash.com promises to search for lower prices and re-book your car rental when rates drop. Basically, they take the work out of the equation and do the heavy lifting for you. Add your rental to Autoslash.com to get started. From there you’ll receive notifications if prices drop. Once they do, you can cancel your original booking and re-book at the lower price.

Join a rewards program: If you find yourself renting cars from the same handful of companies over and over, you can benefit by joining their free loyalty programs.

With Hertz Gold Plus Rewards, for example, you can score a domestic, one-day weekend rental for less than 600 points.

Get a travel credit card: Certain rewards and travel credit cards make it easier to earn discounts with your favorite car rental agent. Hertz grants Citi card members a 5 to 20 percent discounted rate, for example. Citi card members can also enroll in Hertz Gold Plus Rewards for free.