I've tried the plastic rings which rest on the batter heads, also the 4mm rubber which cover the whole head. Neither does a good job in making them quiet enough for practising.
In the end, I glued the 4mm rubber onto 4-6mm plywood, cut to fit each batter head, and that worked rather well, and could even be turned over for a different feel and sound. (Thinking about marketing the product!)
But - I'm at a loss as to how to mute cymbals - tried the same 4mm rubber, and the 'covers' which go over the edge of the cymbals, unsuccessfully, and the most problematic is the hi-hat.
So, to the question - what other methods have you used to damp the sound of all, or any of the drumkit (that have worked or not!), particularly kick and hi-hat?
I'm aware of a practice kit produced by Bill Sanders, and do have an electronic kit, but would prefer to use a standard kit already set up in the studio. Practice pads are o.k., but to practise moving from drum to drum/cymbal, etc, they're hardly the solution.