Small and medium-sized magnets can usually be separated by hand by sliding the end magnet off of the stack. Medium-large magnets can often be separated by using the edge of a table or countertop. Place the magnets on a table top with one of the magnets hanging over the edge. Then, using your body weight, hold the magnet(s) on the table and push down on the magnet hanging over the edge. With a little work and practice, you should be able to slide the magnets apart. Just be careful that they don't snap back together once they become separated.  For very large magnets (generally 2" and larger), we use a specially made magnet separating tool. You can see pictures of one of these tools as well as instructions on how to build your own on this page: Build your own magnet separating tool.
For a more in-depth explanation, check out our article: How to Separate Strong Magnets which includes a number of short videos.