Picture yourself in the sound field. Mixing in 5.1 creates a sound field that surrounds you, as opposed to stereo, which creates a sound field in front of you. When mixing in stereo, you typically mix as if the performers are in front of you. In 5.1, you can mix as if the performers are in front of you, behind you, beside you or all around you. You need to decide your sound field approach before you begin your mix so that you have a plan on where each element will be located.
Pan your instruments in the surround sound field. While stereo panning is between two speakers, you use five speakers when mixing in 5.1. There is a center channel, a left-front channel, a right-front channel, a left-rear channel and a right-rear channel. A surround sound panner allows you to place your instruments in any of the five speakers, or in a combination of adjacent speakers. Use the surround panner to center sounds around the listening position.
Add width to your tracks. Think about a 5.1 speaker setup as multiple stereo setups. You can get width on the sides and from behind, not only from the front. Use this to your advantage to make multiple instruments sound really wide, such as drums and piano. Spread your stereo tracks to two or three speakers to make them sound huge. Be careful not to spread your tracks too much, though, as your stereo tracks can sound like separate mono tracks when they are panned too wide.
Use the center channel. In surround mixing for video and film, the center channel is used for dialog. For music, you can use the center channel for many things. Try putting key elements like vocals or solo instruments in the center channel. You can also try using the center channel to anchor your sound image. This works well when you have moving elements that can otherwise be disorienting.
Kick some bass through the subwoofer. The "0.1" in 5.1 stands for the subwoofer. If you are mixing for 5.1, you can expect that the end user will have a subwoofer. Don’t be afraid to use the LFE (low frequency effects) channel feeding the subwoofer to bolster the low-end content in your mix. Be careful not to get too carried away with the big bass, though, as bass frequencies can eat up your dynamic space quickly.