I want to make dj mashups - How Do I?

[Ferocious Q]

I've always wanted to combine songs (kind of like the DJ Hero Activision mixing team). What programs could I use to make music mashups? I've tried Audacity, but it fairly doesn't work as well as I'd like...

I also thought that the songs "Lights And Sounds" by Yellowcard and "Helicopter" by Bloc Party could sound interesting together...

[Ferocious Q]

http://www.youtube.com/watch?v=CVmPki7sAZA

Here is something that I like to attempt to make. It's a great mashup.