What to put under speakers on a ceramic tile shelf

I have a pair of B&W 602 bookshelf sized speakers. I have them placed on a ceramic tile shelf. What type of material (rubber feet?) should I place under them for best performance?
Try Viprapods. It's a different approach from the standard cones or points but I find it sounds more open and dynamic. Plus it won't mess up your marble. I just put some in and that stuff is pricey!
A follow-up to Maxgain's suggestion: if you decide to use Vibrapods, be sure to put something between the Vibrapod and the ceramic tile. Over time, Vibrapods will stain many kinds of materials they contact. Something like a small, thin plastic disk, or even a piece of Zip-lock bag material, will suffice. (I used Vibrapods on an oak shelf, for example, and they permanently stained the wood.)

The other choice which has not been mentioned is an adhesive like Blu-Tak. Blu-Tak, however, is relatively expensive -- about $10 for a pack. For about $1.00, you can buy the same stuff at Office Depot under the brand name "Tak 'n' Stik", manufactured by Elmer's and other companies. I use "Tak 'n' Stik" under my surround speakers and center channel speaker, and find it works very well.
My experience suggests Lak is right.

When it comes to setting up various speakers on ceramic tiled floors - I have tried everything known to man (only about a third of what is known to woman). Brass has often been best, particularly with panels. With dry speakers like Thiels, thin nylon washers were very good. I have found blu-tak can be good between lively little monitors and light steel stands (spikes are better with heavy sand-filled steel stands), but is not so good betwwe speakers and ceramic floors.

Anything you can do to ensure the shelf is extremely rigid will be beneficial.

Vibrapods have various impacts with speakers, depending on the weight of the speaker and the grade of Vibrapods used - ie. the resonance point. But I have always found that Vibrapods present a trade off where they improve most aspects of the sound, yet have an achilles heel somewhere that is unacceptable - again, depending on the resonance point. The sonic effect depends obviously on where that resonance point falls. So some music will sound better, and some will sound just plain bad.