What's the best way to transition from fast to slow guitar riffs, vice versa?

I'm trying to write some Black Metal/Sludge songs. I can come up with
some decent Black Metal parts, but transitioning from a faster tremolo
sound to just mammoth slow riffs sounds a little odd to me...was
wondering if there's a good way to smooth such a transition, or some
help on writing sludgy/doom riffs.