If it's just me, au natural, with nothing but a fantasy, I prefer to be under the duvet (nice and warm), on my back with a wet wipe ready in my spare hand :)
If i'm using a toy, i'll probably be outside the covers to stop the lube covering everything. Position will depend on the toy, but I mainly use sleeves on my back.
If someone is watching on cam, it can get a bit uncomfortable positioning to give them a good view. I use lots of pillows and limit the toys to a monkey spanker or stroking with a vibe. I have a black "camming" bedspread to provide a good contrast for ejaculation.