This is more of a statement than a question.
I've quite a few people ask this question and as a front desk agent for a luxury hotel the answer is yes, absolutely.
Many people don't understand that we delegate where you sleep at night, we can move inventory as we see fit and complimentary upgrade at our discretion, but let's unpack some common misconceptions.
"the best way to get an upgrade is to smile and ask for one".
FALSE - when you go for dinner at a restaurant, do you say "I've never been here before, is there any free deserts available?" or "I was wondering if you had any free appetizers available?". No, of course not. Most agents will tell you no, but you may get lucky sometimes. It's the most annoying question though because what did you do to deserve an upgrade? Show up?
"The front desk staff will lose their job if you pay them cash for an upgrade"
FALSE - hotel management even admits to stating that they would give upgrades for cash. They're rooms, not retail items.
"check in early and you'll get an upgrade"
FALSE - The hours of operation in the hotel industry seem to be negotiable to some. They're not. Checking in early and asking for an upgrade is quite possibly the #1 way to not get one. Not only are you showing up before check in time wanting your room, you're also asking for an upgrade. That's like showing up to your dinner reservation 20 minutes early and asking for the best seat in the house.
"you can pay the front desk and get an upgrade"
TRUE - 110% true. Every agent I work with, every hotel I've worked at, every front desk will tell you the same thing. If you paid me $20 I would ensure you had the best room within my power. The agent delegates your room and can block it and prevent it from being moved, usually with no questions asked. We have guests that will pay the agent every time they stay and they've had an upgrade every stay for the past 10 years.
"my attitude can impact my upgrade"
TRUE - I've had guests check in who were complimentary upgraded to a private floor due to a high rate and because they checked in early, refused to speak, tossed their credit card on the counter (instead of handing to the agent like a human), and acted entitled, we had downgraded them from their complimentary upgrade and put them in a noisier room while making them wait 4 hours until check in time. This guest didn't even realize that they had just lost a $800 upgrade due to his attitude.
"if I'm nice I'll get an upgrade"
TRUE-ISH - this is a iffy one. It depends on the agent. Usually if there is space and your talkative and nice you'll get a nice room. Money is always a sure fire way. Agents appreciate nice guests, making eye contact and talking. Things like handing your card over instead of throwing it can make a difference too.
How to give a tip?
Easiest way, stop by earlier, don't ask to check in, ask to check your bag while you go out. Ask for directions for anywhere, after the agent shows you, compliment their service and tip them. $20-$50 will get you very very far as of these upgrades are $100+/night, $20 is fraction of the cost.
Either that or just give it to them.
So from an agent at a luxury hotel, yes it works and will for every hotel.