Getting a fourth night free when you book a three-night stay at a hotel is a pretty sweet -- and nowadays, pretty common -- deal. In fact, hotels are also offering food credits, spa discounts, even gift certificates to Amazon.com (see some deals in the table below) all in the hopes of luring travelers to fill their empty rooms.

Yet the deals aren't so rewarding when it comes time to check out. Hotels tack on fees for everything from receiving a package to using the Internet -- all of which can really add up. Indeed, the industry collected $1.75 billion in fees and surcharges in 2008 and in 2007 -- the highest amount to date, according to Bjorn Hanson, professor at NYU s Tisch Center for Hospitality, Tourism and Sports Management.

Hanson predicts that figure will decline in 2009, however, but mainly because of decreased occupancy. Those irksome charges aren t going anywhere.

To avoid the shock of a $100 early departure fee or $14 charge for using the Internet, travelers need to do their research well before they reserve the room. Here s what you need to know before booking your next trip.

Don't assume anything is free

That bottle of water and bag of chips next to the bed probably aren't gifts from the hotel manager. Before you take a sip, ask the front desk if there s a charge. And while Wi-Fi access is an increasingly ubiquitous amenity at most hotels, there s often a charge ranging from $10 to $17 a day to use it. If you know you ll need to get online and don t want to pay, look for free Wi-Fi hotspots in the area or seek out a hotel that offers it free.

A change of plans can cost you

Arriving a day late for check-in or deciding to return home early can result in significant charges. At the DoubleTree Club Hotel in Las Vegas, for instance, guests who leave prior to their reserved departure date get $100 tacked onto their bill or the equivalent of one night s stay plus tax, whichever is less. The best way to avoid getting dinged: Notify the front desk as soon as you know your change of plans. To be on the safe side, ask the hotel before you arrive what their cancellation policy is.

Beware of the disclosure gaps

While taxes and resort fees are commonly listed when making a reservation, other charges based on guests transactions like making phone calls and sending a fax are not as evident. The big travel booking sites like Expedia (EXPE) and Orbitz (OWW) don t include comprehensive lists of surcharges when you book a stay and hotels aren t always so forthcoming about them on their own sites either.

When booking a reservation at the Westin San Francisco Market Street in May, for example, the site quoted a room rate of $148 a night, including taxes. What s less obvious: The fact that checking email costs $14.95 a day and parking will set you back $48 a night. Leave a day early and you ll get slapped with a $50 early departure fee. (If you prepaid for your entire reservation, of course, you get no refund.)

Lack of fee disclosure is so common that one hotel has turned it into a marketing gimmick. This week, the Sheraton Chicago announced a No Hidden Fees package valid through Sept. 7.

Be proactive and dispute charges

When making a reservation, either through the hotel s reservation desk or a travel agent, ask if there are any automatic fees that could be charged in addition to the room and standard taxes. Feel free to grill the reservationist and ask for his or her name or identification number. That way, if you get hit with a fee you weren t aware of, tell the front desk who you spoke with.

If you do get charged -- either for a service you didn't use or were told you wouldn't get charged for -- politely ask the front desk to remove the charges. You ll have better success disputing a charge in person than over the phone later. Just don t expect too much clemency from the staff. These types of fees are waived less frequently than they used to be, says Hanson.