I was just wondering what everyone's approach is to doing a remix or a mashup?

I've been dabling in it just now but would like some tips on how to go about it.

Tips like:

What software people use.
How do people break down the original to use pieces of the track.
Where do they get samples, acapellas etc.
Other must do methods.

Anything at all would be great.

Acapella wise I usually go to Acapellas4U but are there any more out there or even instrumental sites?