Clenching while in posture

Body: 

I've just realised over the last few days that when I'm standing (not walking) in posture, that I'm clenching my upper thigh muscles (backs of thighs), bum and muscles in my lower back just above my bum. I am not good on the names for these muscles at all - forgive my ignorance. Anyway I'm pretty sure that this clenching means trouble? Am I over compensating for a weakness elsewhere perhaps? Should I clench to keep in posture? Worried about doing damage to my pelvis unknowingly. I have had lower back problems (short phases of acute pain once every few years) and think my core strength is poor but was wondering if anyone could shed light on this and any exercise that might help?
I'm consciously relaxing said muscles every time I notice that I'm tense but this is literally every time I'm standing!

Forum:

I find deep breaths in child's pose can release those posterior muscles and might help you to be more aware of when you are unintentionally clenching. There's also a page in the Hips book about shifting your position when standing. I just read it this morning. I'll try to find it. - Sierra

Page 87
"When standing long periods of time, turn your toes out 15 or 20 degrees and place one foot in front of the other. Shift your weight every few minutes by placing the opposite foot in front.

I thought it was interesting when I read it. I haven't applied it yet. - Sierra

I'll give both of these a bash!