What is the correct way to prevent MacBook Pro (mid-2010) from sleeping on lid close?

Question

I have MacBook Pro 13" mid-2010 (MacBookPro7,1), and sometimes I need it to work with the lid closed (long compilations, etc. while I'm on the go). I have tried InsomniaX, but it has no effect. I enable it via the menu bar icon and it displays "enabled", but, if I close the lid, my notebook will go to sleep in a dozen seconds or so. Any way I can prevent this? I know about the heat issue; it doesn't bother me.