
ANSWER - For me this is a classic case of practise makes perfect. The spell I do the most frequently is the one I am really good at. And its a strange skill. I spend a lot of time on public transport and I loathe and detest people intruding on my personal space. So I envelope myself in an invisible shield to prevent people sitting next to me or near me. Now I have to be really concentrating to make it work and I get cross when I forget and someone plonks themselves down next to me. But when I work at it the results are spectacular, the entire bus can be full except the seat next to me and people will choose to stand up rather than sit down next to me. (Lol, I do sometimes wonder if being somewhat of a wide witch there isn't that much space next to me and that plays a part in it....)
And I'm pretty good at sending out subliminal suggestions as to where people should sit, I amuse myself by mentally directing them to the seat I want them to sit in. So I'm sorry my most powerful spell isn't something a bit more glamorous but you know what, I don't think I'd swap it, its far too useful to me when trying to avoid people with their headphones up loud or those annoying people who use their mobile phone on the train. (I'm working on a spell to turn their phones off, I haven't got anywhere yet but my dream spell is to remotely explode other people's mobiles. I'll let you know if I ever get any success with it!)
Image http://www.sxc.hu/photo/1329547 (Mzacha)