With this exercise you want to make sure that you are on a stable and heavy enough bench. Feeble benches may tip to the side if your balance is not perfect. Thus, pick your benches wisely. The best type of bench to use is in fact the barbell bench press bench. This bench is guaranteed not to budge.
Also, for those of you that want to go heavy on this exercise, it's very useful to have a spotter to help you squeeze out those ever-important last few reps. If you don't have a spotter you can manage but you might not be able to get the ultimate burn-out on your last few reps.
Last but not least. Don't bang the dumbbells together. This exercise should be done in a smooth and controlled manner. Slamming the dumbbells into each other at the top means your are not giving your movement the proper "dosage" of power and control.