I usually meditate in bed before I get up, so I am laying down, but I try to prop myself up a bit so that I'm not laying flat, else I fall back to sleep.
Such a coincidence that this question came up when it did. I've always meditated lying down as that's the only way I found relaxing (my lower back hurts sitting up for 10 minutes which is a separate issue). Lately though I've been getting up early and since I meditate in the evenings, I found myself drifting off when I lie down. So will try sitting half up today. Think if you're not falling asleep and feel most comfortable lying down, then that should be fine
It depends what I'm trying to get out of it and what time of day it is. If its the morning, or I want to feel energised after, I do it sitting. If it's before bed and I want to relax, I do it lying down.
I have started meditating by sitting on a pillow on the floor. I find this helps me focus on proper posture while breathing.
You can choose either but my first one is lying down. Instead of pressing snooze for 10 minutes I use that time for a mediation session which really gives me a good start to the day.