How do you appreciate something before it's gone?

Imagine you never had it in the first place and how that would make you feel now

I'm not sure I completely understand the question. It sounds a bit like "How do you enjoy a nice meal" - there's no way to instruct someone how to appreciate or enjoy something, you just have to take a moment.

The thing with the "before it's gone" part is that we don't always know which things are temporary, so you just have to cultivate an attitude of appreciating things in your life, including the people in it.

Just think what it would be like to lose it right here and then. This is what works for me.

Try to briefly realize it won't last. Unfortunately most people can't do this, at least in western societies. We're doomed to only realize what we had after it's gone.

Imagine its gone

Maybe spend time with it without expectations. Just fully take it in and notice little things about it. Notice how it really is and how it makes you feel. Maybe like this?

Gratitude journaling actually works. Top cope if you need to swing the pendulum the other way.

I'd like to take things a bit more for granted, myself, but my damn mindfulness habits make it hard

Take a moment to think about the good things you have. Bring this state of mind in your daily ventures. Notice more good things you have not thought about before. Imagine life without. Think of people living without them. Appreciate that you have them.