My favorite material is Sorbothane. Audioquest sells (or use to anyway) it in sheets with adhesive on one side. It can be cut to form pads that you can rest the speaker on. The adhesive is unnecessary for this application, but it never seems to cause a problem. Stick the adhesive side to your stand. The Sorbothane will stick to your speaker bottom when you place the speaker on it, but it can be removed without damage. I have never had any speaker finish damage using this method. If you let your speakers sit on the stands, after a while they will be stuck as if glued (more like suction though). No need to worry, just apply moderate pressure on top of the speaker - attempting to tilt it while holding the stands steady (I usually stand on the base of the stand). Use your body or arms to prevent the speaker from falling if it realeases suddenly - this is just a precaution, when moderate steady pressure is applied, the speaker releases slowly. Be patient, and the Sorbothane will release after several seconds of steady pressure. The Sorbothane may end up on the speaker or the stand, but it is easily peeled off if you go slowly.
I have found that Sorbothane is better than any other method including spikes and blu-tack. It connects the speaker to the stand very solidly and has never caused any damage to a speaker I have used it with.