Setting up your Amp for the Stage - Part 1
#1 - Setup your amp off the floor
There are two purposes for having your amp with you in performance...
1) To achieve the tone you want
2) To hear yourself on the bandstand (musician's amps are known as the "backline")
If you set the amp on the floor, you're pointing your amp... your monitor... at the back of your calves. Last time I checked, our ears are on our heads, which is 6' up for me. Simply put, if you can't hear yourself, you can't play. Get a chair, two milk crates, your bass or guitar player's road case, a ledge on the stage... whatever is available and set your amp on it so that it's closer to ear level.
Depending on the stage, sometimes it sounds better on the floor of the stage, but you'll have to experiment with this when setting up. This is also only the case with an amp large enough to transfer vibration to the stage (such as a Bassman).
Depending on the size of the stage it can also be difficult to get the amp up without impeding the view of another musician. In this case you just have to compromise... maybe just one milk crate.