I use stuff called Quake-Hold. It's basically the same as Museum Putty and Blu-Tack. It's a thick, waxy substance. You take some out of the jar, roll it into little balls, and stick a ball under each corner of the speaker. Then you align the speaker on the stand and press down. Works like a charm!

It's supposed to be safe on all finishes, and any residue it leaves behind can be cleaned up with a damp cloth.