Sail7seas said, "I meant to add that most people tip by putting cash into an envelope and handing it to the steward. We always put our name and cabn number on it and usually write a note of thanks.
I completely agree. If somone has given service worth of a gratuity, it is only courteous to thank them both personally, as well as with a brief note of thanks. And by putting your name and cabin number on the note, you make it far easier for the employee to know whether your tip is in addition to the tips left on the shipboard account (and therefore do not have to be pooled), or in place of them (and subject to the pooling rules).
Leaving cash on the desk or bed with no note, is not the nicest way to thank somene for their hard work (although nicer than not tipping at all). Still it only takes less than a minute to write a note that says "Thanks for your hard work and excellent servide. You made my cruise special!", signed Your Pax, cabin number xxx.