Heres a few things I do
say you have a sustained pad sound, set up a bus with one or two reverbs on set to 100% wet, turn your pad all the way down and use it as a send effect.Then use the send volume on the pad as the volume which should give huge space to your sound, then you can use the volume to bring the pad a little more foward in the mix.
Layer lots and lots.
Pitch shit down lots.
Duplicate a pad sound(helps if its audio for this), pan one of them hard left and the other hard right, reverse one of them or pitch one of them down a good few semitones(this gives a huge wide sound thats also pretty easy to fit in the mix)
Use low pass filters to push things further back.
Of course it doesnt have to be a pad sound, you can get an ambient pad effect of anything with enough pitching down and verb, I use hats,claps speech, synths, drums, whatever.
Absynth is great for atmospheres as is FM7.

Get stoned and make em at 3 in the morning: )