Getting enough sleep is important as well. Anecdotally, I have experienced subpar lifting performance when I average less than 8.5 hours/night for a few days.

I also plateaued on Squats during SS after about 4 months, so I lowered my work weight to 80% during my middle squat day (Wednesday in a MWF routine), as Rippetoe suggests, and I immediately broke out of it.