How to avoid stripping screw heads?
What am I doing wrong?


If you're stripping the heads, Phillips head screws probably aren't the right kind for the job.  They cam out under excessive torque [1].  And once they do, the bit becomes a grinding tool, chipping away the head.  They're good for uses where high torque isn't required.  Phillips head screws intended for use where they might require some torque are hardened, which reduces the chipping if they cam out.

The solution is to use the right kind of screw for the job, and to not substitute brute force for proper technique in a demanding situation.  Excessive torque shouldn't be required if you don't cut corners on technique.  But if a lot of torque is needed, use screws that can handle it.


Drill a pilot hole.  There are actually different optimum-sized pilot holes recommended for different materials.  With soft wood, a precisely sized pilot hole often isn't critical if you are close, but If you are screwing into hardwood or metal, the right size pilot hole is important.  
Lubricate the threads by rubbing the screw on some candle wax or bar soap (not liquid soap), before you drive it.
Don't use a worn driver bit or one that is not the exact size for the head.  This is especially important for Phillips head bits because camming out and grinding away the head also grinds away the bit, making it more prone to cam out.
In a demanding situation, use hardened screws or a different type of head.  For example, square, hex (Allen), or star (Torx) socket heads are common for high-torque applications, or a hex head that you use a socket to turn.




[1] There's a long-running argument on whether cam out was an intentional design objective or just a characteristic of the design.  According to Wikipedia:  The design is often criticized for its tendency to cam out at lower torque levels than other "cross head" designs. There has long been a popular belief that this was actually a deliberate feature of the design, for the purpose of assembling aluminum aircraft without overtightening the fasteners. Extensive evidence is lacking for this specific narrative, and the feature is not mentioned in the original patents. However, a 1949 refinement to the original design described in US Patent #2,474,994 describes this feature.