When I do it I do both and concentrate on the actual visualizations more I think.
From what I have read of the MILD method, the visualisation and the mantra are designed to go hand in hand. When you visualize you visualize the last dream you had until you contact a dreamsign, then you visualize yourself becomming lucid and then repeat.
I believe that the idea is that you fall back into your last dream, excpet this time (as you have been rehearsing) you recognize the dream sign and become lucid.
But I guess part of the idea is to adapt any technique so that it works the best for you.
Ow and no I cannot get it to work for me on demand.
ypm.