Many regular visitors to Las Vegas see the $20 trick as their first real gamble when they arrive in Las Vegas. The way the $20 trick works at your hotel as you are checking in, when the hotel desk employee for you credit/debit card and identification slip the $20 bill between the two items; As you are handing the items over ask if there is any free upgrades, most likely the employee will start typing away looking for an upgrade for you.
If the hotel clerks boss is nearby they might just hand the money back but most of the time they will look for upgrades which could include things such as a Strip view, pool view or even being upgraded from a regular room to a suite.
If the desk employee finds an upgrade they will tell you and if you agree then keep the $20, if it’s doing the week or non peak time you will have more of a chance getting an upgrade. I wouldn’t recommend trying this at a motel because being upgraded to the room nearest the ice machine might not be worth the $20.