Depends -

Hogmanay at a fourstar hotel - kilt and Jacobite shirt.

Usual gigs - jeans, cool shoes (for me hot feet) and often a light cotton collarless shirt over my waist band from Skye Batiks - wacky colours and designs. I wear the shirt out so that belt buckles and studs on the jeans don't damage the guitar. I loke collarless shirts because they don't get messed up when you change instruments during a performance.