Don't use the phone.
Most people know by now that hotels charge an arm and a leg for even the shortest of local calls. A lot of people think they can get around this by bringing along a calling card. Forget about it. You will be charged for every second your room phone is off the hook, unless you are calling within the hotel.

The biggest mistake you can make with the hotel phone is to take up those credit card direct dial offers that you often find by the bedside table. Avoid these at all costs! Not only will you be charged by the hotel per minute for the supposed "toll free" call, weeks later you will find out that those calls can cost up to five US Dollars a minute.

Don't use the minibar.
Don't be fooled into thinking that you can swipe a drink form the minibar and replace it with a store bought one before they check the stock. Hotels have cottoned onto this long ago and often stock the bar with cans and bottles that are not available in local stores, so you end up paying $5 for that cola, which is smaller than a store bought can anyway.

If you want to store your own items in the minibar, providing the minibar isn't the type where charges are levied automatically if things are removed, you should remove all of the pre-stocked items, put them on a nearby cabinet, making it clear that you haven't consumed anything, and then put your own stuff in there.

Take advantage of lounges.
Many hotels nowadays offer exclusive "concierge" or private floors which, for a relatively small price, you can be upgraded to one of these floors and have access to the lounge where you can imbibe free evening cocktails as well as a free breakfast and in many cases, free tea and coffee around the clock. An example of this kind of lounge is the Grand Club at Hyatt hotels. You can ask for an upgrade or a reduced price to one of these rooms at check-in.

Join hotel loyalty programs.
Innovative hotel loyalty programs like the Starwood Preferred Guest program allow members to redeem points for not just free rooms and room upgrades, but also for things like free breakfast, or unlimited local calls, or sports club access and many other things. Considering that if you have a co-branded credit card that earns points for every dollar you spend in your day to day shopping, you could conceivably get all these extras for free. With all loyalty programs free to join, it is certainly worth looking into.

Other services that will quickly add to your bill are things like laundry, parking and asking the concierge to book or suggest anything for you. You can be almost certain that any shows or tours that the hotel recommends will have the hotel's kickback included in the price.