if you get very underexposed images it's probably because you're not keeping the aperture open by holding a lever in the right position. I used to keep it in place with a piece of paper, also it seems some fancy adapters can be useful to the same purpose.

I've not tried this extensively, still enough to notice it's really easier with a slightly longer lens; if you have a kit lens or another all-purpose zoom (usually around 18-55 for cropped sensors) it gives decent results at its longest end.
If you read photojojo's tutorial you likely noticed they suggest to move the lens very little from its usual position, this is important to get things in focus and to avoid light leaks.

sorry for the bad quality of the image.

I removed from my answer a couple lines of mild criticism some seemed to find offensive.
I still advise against freelensing (as more than a brief experiment) because it leads to good chances of damaging both the camera and the lens used.
I also think, as answering a request for advice on this matter, it's fine to state I stick to my most (apparently) controversial opinion (anyway already clear if one reads the comments), i.e. not considering freelensing a proper photographic technique, because

• it makes permanent gear damage easy
• it gives little to no control over the technical basics of photography
• it doesn't allow for basic consistency in results

hope this made my point clear in a polite way.