A knee pillow. I have always struggled with the occasional back pain because I would sometimes twist my legs/back at night and be in pain the next day(s). A knee pillow has massively changed that for me. I pretty much never have issues with my back caused by sleeping weird now.
In a similar vein, nasal strips. My nose often gets obstructed at night and it caused a ton of sleepless nights because I had a hard time breathing. Finding the strong nasal strips I'm now using pretty much eliminated that problem.