It Depends, Really.
When I miss my grandmother, I fiddle with the ring she left me and talk aloud. I know she hears me.
When I miss a friend, I check in on them. Whether they know it or not.
When I miss an ex-love interest, I evaluate. Reminisce on why things went the way they did. Not drowning in my sorrows, mind you, just not forgetting. Was never good at that.
When I miss my current love, I look around the room. There are traces of him everywhere here in the space we exist in together.
When missing a love that is far, far away....I've been known to do several things. Write poetry. Drink milk from a saucer. Look at photos. Read letters....
It all depends on who it is I'm missing.