The $20 trick is essentially a bribe to the front desk clerk you are checking in with in an attempt to get a better/upgraded room/suite than you normally reserved but at the price of the room you originally booked.

There's a thread in the LV forum that discusses it and a even a website devoted to it but but there are different ways to go about it. I usually say we have a basic room booked but were wondering if they could check if we could have a complimentary upgrade to a high floor with a nice view of the strip. If I'm fairly confident that I'll get an upgrade (ie. suspect the hotel is not fully booked), I'll tip the $20 immediately. Otherwise, I'll have my wallet open on the counter with a bill ready to go to let the clerk know I'll tip him if he can accommodate our request.

A basic upgrade at the Wynn and Encore is usually pretty easy. By basic, I mean a non-view room to a high floor Panoramic view room of the strip. I think you'd normally pay an extra $40/night or so for a Panoramic room.
However, if the hotel is really booked up, it might not work or the upgrade might not be as great.

We're 2 for 2 for upgrades at the Wynn / Encore. I tipped immediately at the Wynn while the wife tipped at the end during check-in at the Encore. In the past we always ended up with crap rooms with a view of a parking garage or whatever . Every since we started asking and employing the tip, we've gotten great rooms every time except once during a stay btw Christmas and New Years when the hotel was packed. The main thing really, is to ask nicely.

BTW, if you're going to book a Concierge room, I'm not sure how much value you'd get employing the $20 tip.