How do you tip?

My wife and I have had a long-standing disagreement about whether it is better to leave a tip on the table, or add it to the bill. (Note that I am not touching the question of whether one should tip at all. Most wait staff make so little salary-wise I have no problem adding to the pot.) There are pros and cons for both sides and things seem pretty evenly matched. The major points are:

I think that adding it to the bill is just simpler. Besides, who carries cash around these days? Heck, you don't even need cash for Tim Horton's anymore!

My wife feels that leaving it on the table ensures that the waiter/waitress gets the tip. Of course, that may not happen if he/she doesn't clear her own tables.

Well, guess what? I am changing my mind, but with a twist. The other night we were out for dinner (New Year's Eve, in fact) and the waitress we had was just super. She had a genuinely happy smile and a "Happy New Year" for everybody. She made sure all her customers knew who she was and what she was doing about their order any time she walked by. If there was a delay she made sure the customers knew that she knew and was looking after it. In short she made the whole experience a very enjoyable one.

It occurred to me after we finished that neither adding the tip to the bill (even a generous one), nor leaving it on the table would do. This woman had to be personally acknowledged. So, from now on, any one who deserves a generous tip will also be thanked personally.