It is vital to protect your fingers when using a slicer. Slicers come with hand guards which have little teeth which grip the vegetable. These are the best thing to use with a slicer. To increase the effectiveness of the guard, slice a flat surface on the vegetable for the guard to grip.
Cut-resistant gloves are another protective measure you can take. They are not cut-proof, but are very helpful. You can wear the glove when chopping vegetables and cutting meat as well. I am wearing a Microplane cut-resistant glove in this photo.
On Cut-Resistant Gloves.