Plot Holes and Devices (And How to Avoid Them)

In all seriousness, though, I do have some advice on how to avoid plot holes. 

1. Outline, outline, outline. I still don't understand why people don't do this. It's like going on a long road trip without a map (Or, let's be real, a GPS; it is the 21st century). You can't see the holes in your plot if you can't even see your plot.
2. Double-check your outline. You have to be sure that your map isn't missing pieces. Don't rely on a map with the Kansas River displayed as a lake in Idaho, or on a GPS that thinks you're in Peru (my mom's GPS did that once). 
3. If you have magic or advanced tech, make sure it makes sense, and has flaws. If your MC has been granted, say, a ring that contains the power of the universe, there has to be a way for the ring to fail, allowing the antagonist to gain an advantage (because all villains need an upper hand at some point in the story). Perhaps the ring's wearer doesn't know how to control the ring, and ends up putting themselves or someone else in danger. Or perhaps the ring tries to take over the host's body.Either way, you need to make sure the ring (or whatever magical/advanced tech item) has limitations, flaws, and won't have your reader asking, "Why couldn't s/he just use the ring's time travel capabilities to save [insert important character]?"
4. Also, make sure your worldbuilding makes sense. You can't have your half-giant biker tell your protagonist: "You're a wizard, Larry!" out of the blue, without leaving traces of his wizard-ness, like having him talk to a snake or making the glass of the front of said snake's enclosure disappear. Make sure you don't drop a plot-bomb without letting the reader see the plane dropping it, if that metaphor makes any sense.
5. If I forgot anything, tell me. Also, check out this bit of totally unrelated advice that I really should be putting into a different post.  

Happy plotting!