To avoid stiffening up, I play music (usually progressive house or something similarly rhythmic-but-not-too-distracting) into noise-cancelling headphones and let the music move me. Because I'm not standing still, I don't experience lower back pain, stiffness, soreness or anything else.
If I had to go back to sitting now, it would kill me.
[] NB I wrenched my back last autumn by twisting in a funny way while painting a ceiling with a roller on an extension pole. After a couple of weeks of Advil and an aggressive regimen of back exercises and stretches, my back pain was completely gone and I feel better than ever.
I've been doing the standing desk for about 4 years now. I just bodged something together for my setup, but I'm thinking about investing in a VariDesk system instead. When I sit, I'll probably use an exercise ball instead of a chair.
Then I developed pain in both knees, worse on the left knee. I figured that it was because of too much sitting, so I got a standing desk. In about a month, knee pains gone, completely.
Never even bothered to go see a doctor.
After standing for too long, I started having pains in my lower back. Solution? I got a bar stool, so I now alternate between standing and sitting.
What I learnt is that our body's joints (knee, elbow, waist, ankle, etc) were not designed to remain in one position for too long. Movement lubricates them.