Maybe this won't help anyone but me, but when I figured it out today it was a lightbulb moment and I'll never forget how to convert mg to kg (or whatever). Here goes.
The left hand is viewed as "the big number" (kg, for instance). The right hand is viewed as "the small number" (mg, for instance). I remember this because it's kind of backwards for me as I'm right-handed. Like I said this may not make sense to anyone but me, I'm kinda wacky. Follow along...
If you wanna convert a BIG measurement <waves left hand around> to a SMALL measurement <waves right hand around>, you move the decimal place from LEFT <punches outward with left hand> to RIGHT <punches outward with right hand>.
If you wanna convert a SMALL measurement <right hand> to a BIG measurement <left hand> you move the decimal from RIGHT to LEFT.
It's all in the hands, baby. Once I got the Left=Big, Right=Small thing down, I had no problem remembering which way to move the decimal.
Wah-Lah. I sat there doing metric conversions today, waving my hands around and getting 100% on my quiz. How cool is that.
I had seen various explanations of the concept, but it wasn't until I "visualized" it with my hands that it actually STUCK in my head and was useful. Previously I was always wondering "now which way was it....?"
Hope it helps! :roll