Here's how it works:
Fold a $20 bill in quarters (in half lengthwise and again) and put it under your CC at the check in desk. Leave it sticking out a bit so the clerk can see it. When you present your card for check in, you slide it forward a bit with the twenty and say "Are there any upgrades available?".
Theoretically they will see the money, take the card and the bill, say "Certainly Sir/Maam, let me see what I can do for you." and give you the best room in the place.
Stay thirsty my friends, and drink plenty of water when visiting Vegas, it's dry out in these parts.