I used to do it by looking at the reflections on a deep black crystal I had. There was a certain angle where it would create some very interesting patterns and shapes, warped and upsidedown reflections.
To me it was more of a meditative exercise but it can be useful to do before spellwork. It can also be done with water on a dark bowl, smoke or a mirror like you mentioned.
Kelly Anne Maddox did a video on scrying also: