Ramp up quickly with a difficult puzzle or a mid-level multi attempt for your skill level. If I knew I wanted to practice, but wasn't really feeling it, I would immediately do a 5x5x5BLD solve with no other warm-up.
Also, experiencing a DNF or failure of some sort is a good warm up. So just do a 3x3x3BLD solve, even when your memo won't stick. During the solve you will likely DNF because your memo didn't stick. Try to focus on why it didn't stick. Do you need better images for those cases? Do you need a different memo method altogether? DNFs and mistakes show you what you need to pay attention to or what you need to work on for the next solves (or for your method in general).
If you're really not feeling it, then don't practice BLD in that moment. Go outside, or do something else for a while.