How do I masturbate? However I can! I'm not picky, sitting, standing, laying, crouching, it all feels great!
If I'm not engaging in any anal play I usually just have a good ol' wank while laying on my back before going to sleep or before getting out of bed in the morning. Usually both 😜 It works out pretty well, I blow my load, clean up with an always handy nut rag, then roll over and pass out comfortably.
If I'm enjoying my ass, I usually lay on my left side (more akin to Simms than knee-chest) and work my toy(s) with my right hand. I can sometimes orgasm anally so I usually don't even worry about playing with my dick. If an anal orgasm just isn't in my future I will usually turn on to my back and make a 'bridge', lifting my hips off the bed so I can grab my dick and rub it out. Other times while standing I'll put my foot on something a few feet tall and work my ass & dick that way. Both positions give me pretty good access.
What Bricker said about kneeling is pretty awesome for ass play too, but it usually ends up hurting some part of my anatomy before I get an orgasm. Usually my ankles or knees, depending on what I'm kneeling on.
Doggystyle works pretty well for anal toys too. The angles just don't work well for reaching my cock though. I can (quite easily), but my hand is at the absolute wrong angle for sustained stimulation. I find that I can get toys *very* deep in that position though.