We all want to save money when we travel, but who has time to monitor the constant ups and downs of airfares, hotel rates and car rental prices? You can easily spend more time booking and modifying your travel plans than actually enjoying your vacation. Fortunately, there are a few websites that take the hassle out of travel planning, saving you money while you sleep. Take a look at seven of our favorites. Sweet dreams.See: Shop Your Way to Free Flights and Travel PerksAutoSlash hunts for the best daily and weekly rates across available rental car companies at your pickup location, then monitors the prices and automatically rebooks you if and when the rates drop. The free service also checks the rates at other rental car companies at your location and will rebook you with another company if it finds a lower rate. Even better, the site looks for and applies coupons and last-minute discounts right up until the time you pick up your rental car. No need to pay for your vehicle until you drop it off at the end of your rental period.If you're willing to let someone else use your car while you're away, you'll be able to avoid those pricey airport parking fees. FlightCar, a peer-to-peer car sharing service, currently operates at a handful of major airports, including Washington Dulles International Airport, Dallas Love Field and Seattle-Tacoma International Airport. Simply drop off your car at a FlightCar lot (located about 10 minutes from the airport), get a free ride to the airport and then look for an email if your vehicle has been rented. If it gets rented, you'll receive a cut of the rental fee (you get paid per mile driven based on your car type and year). If another FlightCar member doesn't use it, you'll come back to a freshly washed car and no parking fees. FlightCar covers up to $1,000,000 in liability should anything happen to your vehicle while you're away.It's tempting to hop on a nonrefundable hotel deal when the price is better than the refundable rates. But, what do you do when you can't use the hotel room after all? You put it up on Roomer so other travelers can scoop up your digs. When they do, you'll earn back a percentage of what you paid for the room. Simply submit a few details about the accommodations, such as the hotel name, room type, booking site and confirmation number. Next, set a price (Roomer recommends selling the room at least 20 percent below the original market price) and select how you would like to get paid if the site is able to find a taker. Another bonus: Using Roomer is free.See: How to Find Cheap Flights OnlineThere are a variety of ways to earn frequent flier miles, from online shopping to airline credit cards to actual flights, but Rocketmiles makes it easy to earn up to 5,000 miles per night by booking hotel rooms through its website. While you sleep, you'll earn miles that can later be applied toward future travel with programs like Southwest Rapid Rewards, United MileagePlus and American Airlines AAdvantage. All miles earned through the site go straight into the airline rewards program account you selected at booking, so there's no need to convert or transfer miles from one account to another. Plus, the service is free, and there are no required member fees. Of course, there is a tradeoff: You can't earn points with a hotel loyalty program if you book through Rocketmiles.For those who like to cruise, it's a must to bookmark CruiseFareMonitor. The site tracks cruise prices by specific line, ship, departure date and stateroom category, and then alerts users when the price drops. Unfortunately, not every cruise line can be monitored, but many of the major lines are listed, including Royal Caribbean International, Carnival and Norwegian. Though the service isn't free, the nominal fees can be well worth the potentially significant savings. You can pay $10 for the site's basic "Unlimited" option, which means your vacation will be monitored up until the date of final payment (generally 60-90 days prior to departure). "Pay Only if You Save" is another option in which you pay 20 percent of your cost savings if CruiseFareMonitor is able get find you a better deal. If the site can't score you any savings, there's no charge.Booking your stay at a boutique hotel is as easy as sending a tweet to @stayful using the #TweetStay hashtag with your dates of travel and price range (e.g., budget, mid-priced and premium). You'll then receive a tweet back with hotel deals that Stayful negotiated on your behalf at no charge. If you're not Twitter savvy, you can also head to the Stayful website and bid and book hotels on your own. Stayful shows you the best price it found online, as well as a suggested bid to help you secure a boutique hotel at an even greater discount. Keep in mind: Stayful is best used by travelers looking to stay at an independent, boutique property within the next 30 days.Since you pay upfront when you book an airline ticket, you never really know whether you're getting the best deal. Travel search engines like Kayak can tell you whether you should buy or wait given the current fares, but what if the price drops after you've purchased your ticket? Enter Yapta. After you've purchased your ticket, enter your flight details on yapta.com and look out for an email. If the price of your flight drops 24 hours after purchase or if it drops more than the airline's change fee, Yapta will assist you in getting a travel credit. You don't have to pay upfront to use the site, but it does get a percentage of your savings. Note that some airlines, including Delta, American and United, each have change fees that can cost of upwards of $200, so you may only be better served using Yapta on more expensive international flights.See: How to Earn Hotel Perks Using Social MediaAbout the author: Erin Gifford is a Washington, D.C.-based travel writer and founder of Kidventurous, an award-winning family travel resource. You can follow her on Twitter at @Kidventurous, connect with her on Facebook or get the latest tips and tricks for traveling families at www.kidventurous.com.