When you're making an increase (mid-row), it's basically adding 2 beads instead of one between your "up" beads. When you do the next row, you would add another bead between the 2 beads you added during the previous row.
For a decrease, you are skipping adding a bead between your "up" beads, and pulling the thread tight to close up the gap.
Here is a tutorial with pics from about.com. Hope that helps!