Phillips Head Screw Stripped Out

Get out of jail when you strip the head of that Phillips Screw!

Now this won't work all the time, but if it saves you ONCE, it's worth it.

Tip 1: The Hammer
If the screw is mounted in something that can take the abuse, try hitting the face of the screw with a hammer. Make sure you hit it square and if it's a small screw, be gentle!
If you take a close look at the mangled screw heat, you'll see that the metal pushes UP where it's been displaced/stripped, if you hammer it back down you might be able to get enough purchase on it to get it out.
But read on because you may want to try to use Tip 2 to help remove the screw (as insurance).
I have found this works particularly well on those fussy little abused motorcycle master cylinder cap screws.

Tip 2: Valve Grinding Compound / Paste
Although this stuff is great for grinding the valves in your car or bike cylinder (who does that these days?) , it can come in handy to increase "traction" (friction?) on the screw/screwdriver interface.
A little valve grinding paste on the tip of your phillips head screwdriver could get you out of trouble.

Tip 3: Hacksaw
Depending on access, a hacksaw or a Dremel (Rotary) Tool could let you cut a slot across the head of the screw and allow you to use a flat blade driver to get the screw out.

Although it goes without saying, even if you get the scew out, it's garbage. Bag it and take it to the fastener joint so they can find a replacement (and a spare).

WD40, Heat, Impact Drivers are also helpful in this situation...