Nothing. Just focus on your breath. Then I imagine I am miles above the earth. I am then thankful I can sit, undisturbed, no worries about famine, war, or sickness. So, I guess my answer is i think about gratitude.
I try to make an image in between my mind and focus on that. It may be of a mountain, river or rain and hear the background noise of music, concentrating on my breathing.
I think it depends on the kind of meditation. If it's just mindfulness I try to keep my mind still and not think about anything or focus on my breathing if it is a meta meditation that focuses on feelings of gratefulness I try to think and focus on the feeling. You have to find the kind of meditation that works for you since there are many explore and find one you like.