What percentage should you tip a waitress?

It should certainly be variable depending on service, but I agree that 20% is for good service 

20% waiters are paid very little hourly 

I agree that most waitresses don't get paid much usually is the minimum that the company legally has to pay them however the waitresses at my restaurant are paid 3 to 4 times the hourly ave hourly wage minimum. I do this because they are asked to do a number of other jobs in the restaurant as well as it is a good practice for retention of good waitresses. As far as the percentage of tip goes that's on the customer. I believe it depends on the average size of the bill for the most. Usually if the bill is under $25. If over $25 then I noticed that most people tip more of a percentage. However I still believe that I tip is earned and not assumed... to a certain extent!

Servers are very well paid, thank-you, because of this insane tipping custom. Where else can an employer count on his customers to voluntarily pay his best staff? Canadians tip 15%. I tip 20% because I am a well known restaurant owner. The joke I heard from a New York restaurateur, "What's the difference between an Canadian and a canoe? Canoes tip." All studies so far show that tipping has little to do with actual service but the amount of the tip actually depends on how much the server has impressed the guest with looks and personality. The nation that invented tipping, England, doesn't have a tipping culture as does most of the world. Tipping usually happens where Americans frequent.

I believe the industry standard is 15-20 percent.  For fine dining, I always tip 20 percent, but sometimes more if service was exceptional.  For large parties, our wait staff will put 18 automatically on the bill, but this is a risk in that they might have gotten a higher percentage (but also a lower one). 

Thank you for all your answers! Where ever I go, which I have to admit I eat out a lot I do a good tip whether the service is good or bad. What I notice about doing that is because I go to the same places often to eat, the staff will remember me and that nice tip, I start to get front row seats, where do you want to sit choice, big portions of food, lots of attention. I even been moved to the front line like I reserved my visit. Customers, acquaintances I take out are always amazed how I get everything I want and in some cases get discounted prices or something free. A tip is remembered, especially if it's a good one.  

The minimum should be as much as the HST which is 13%.

That really is a personal choice. We add 18% gratuity to parties of 8 or more. Servers who are very customer focused and take good care of the customer do very well at about 20%, but 15-20% is about the range in our area.

Well you are in the service buissness the more you are nice the more I tip you even the average is 20 0/0 so you have to enjoy what are you doing and do not expect 20/ just because specially today with the millennium always on the phone not attentive to their section ..

20% standard for good service,