The attached article is a very short guide about proper tipping. It also tells who it is not proper to tip, such as flight attendants. The article also gives the proper percentage or amount to tip as well, but the main thing to remember is that the tip is mainly determined by extra services performed by the person providing the service.

For hotels, it does state that the hotel doorman doesn't need a tip for just holding the door open. If you ask them for extra help such as, for example, holding an umbrella over your head from the car to the door, though, expect to pay $2-$5. Give the hotel bellman $1-2 per bag, and more if the bags are hard to carry. And, the hotel valet, $2-$5 for taking care of your car. 

Although the article doesn't mention the amount you should give the hotel housekeeping, I have seen that a $1-5 per day tip (depending on the number of beds you used and the general mess you leave) is appreciated.