PowerPoint has a bunch of shortcut keys you can use while presenting in slideshow mode. A few of my favorites:

• B blacks out the screen and W turns the screen white. Hit the key again to go back to your slides. I find this useful for hiding slides during breaks in the middle of a presentation.
  
• Type a number then hit Enter to jump to a particular slide. For example, typing 5 and Enter jumps to slide number 5. Or, if you're not so secretive, hit Ctrl-S to display a dialog that lets you jump to any slide.
  
• Ctrl-A makes the cursor visible so you can use the mouse to point to something.
  
• Hold both mouse buttons down for 2 seconds and you'll jump back to the first slide.
  

Hit F1 while you're in slideshow to see a more complete list.