Speakers slipping on speaker stands

Grip It shelf liner would work great for this. This is a link to an E-Bay auction so you have an idea of what it is, but you can find it at many grocery stores, Target, Walmart, etc., and it comes in several colors. It just lies on the flat surface without adhesive, and it's rubberized so it prevents slipping.

A little unsightly, but here in shaky California a must:

I use black bungie straps from the top of the speakers to under the stand's platform to hold down my speakers to their stands.

I second weather stripping..cheap and available at your nearest "true value", use that for now to get them stable and tweak later

Out of all these suggestions, I agree with the Blu-tack. This works amazingly well. It is often difficult for me to get the speaker off if it has been sitting on it for a while. Try Fun tack or similiar putty product. Any office supply or drugstore carries it for less than 2 bucks.

If you don't mind working to remove the Blu-tack from the bottom of your speakers and the top of the stands when you decide you want to re-arrange things, then I agree with the Blu-tack or Quake Hold (museum wax) method. If you want something that will keep the speakers from slipping, but is less permanent and will not leave behind any residue, then try the Grip It shelf liner.