Best ways to add depth to a mix

[Hooskter]

You can push a sound very far back using low-pass and high-pass filters, or simply use high-quality reverbs like Eventide Blackhole. With it, you can fly off into space

[Aloysius]

Dig a hole in the ground and put one of your two speakers in it.

[legendCNCD]

+ the tapes you recorded

[sfoster]

Predelay helps, and shorter reverbs for clarity.