Need help with the blender 2.5 smoke simulator

Hey guys!

I’m working now on a explosion of a building and would like to create
smoke like this here: http://www.youtube.com/watch?v=ymvdFhblUwk
and not like this: http://www.youtube.com/watch?v=cjuIHb1PpsM

I’m taking about the density.
In my video (the second link) disolve the smoke too fast and the density even faster.

Please give me some help.

Thanks