Whenever you're going to work on something oily, dusty, or caustic (even if you're going to wear gloves)
first rub in some skin conditioner like Intensive Care or its generic equivalent. It will make clean up a lot easier.
Trim your nails. There is little that is more annoying than trying to avoid snagging a hangnail.
When you work a piece either by blasting, sanding, or wire brush, clean up potentially abrasive residue with a polar degreaser
(like dish detergent or Simple Green) in water -- these are much better surfactants than aerosol types.
Then rinse, and dry with a blast of air. Then finish off with a low- or no-residue water displacer/metal protectant like WD40.
On threaded fittings (especially stainless or titanium) into aliminum always use something --
if you plan on taking it apart again, silver-, copper-, or ceramic-based anti-seize. If it has a tendency to come loose,
a thread-locker like Loctite. If it holds fluid (like a brake line or oil cooler fitting) use teflon tape (there is some disagreement on this).
Hot Rod Magazine recommends a dab of copper-based anti--seize on spark plug threads -- apparently it is conductive.
Make sure that the piston pin is a slip fit in the piston. It should slde through under its own weight.
Otherwise, you'll need a pin-press (quite rare) to take them out. Or drive them out and ruin everything.
Alway assemble the cylinders and rings and lands dry. The first few strokes will seat the rings and running for a minute will lubricate them
Have a matte finish on the piston skirts and a little assembly lube on them.
On the case halves use Yamabond #4 or Golden Hematite, or similar.
Wash your hands thoroughly, then peel an orage by hand. It will get all the grease and mung out from beneath your nails.
Use silicon-based lubricants (either aerosol or grease) on parts that get really hot (like valve stems).
The heat oxidizes it to sand and either seizes the part or abrades it rapidly.
Use an acid-based rust stripper (like Naval Jelly) on any iron/steel that isn't strictly cosmetic. Acids are hydrogen donors and this causes a phenomenon called hydrogen embrittlement in many metals. Among other effects, it reverses heat-treatment, rendering the part a very short working life in high temperature environments. Remove rust by abrasion or one of the new(ish) "organic" rust dissolvers.
Use gasket sealer -- it's already in the gaskets. Maybe a little copper-based aerosol on the head.
And if you just can't resist, some silicon-based goop (or Yamabond #4) on the cover-side on the case.
It will allow the gasket to come off with the cover.
Use sand, glass, or metal grit to blast parts that will come in contact with hot oil.
The particles embed into the surface and are released by the heating of the motor --
then it's off to wreak havoc on delicate bearing surfaces wherever it can find sliding or spinning metal to abrade.
Instead of blasting, use wire wheels or sanding discs on a die grinder.
It's okay to blast the outside of a carefully masked part, which is then carefully cleaned and shot with paint.
The only exception is actual ball bearings (Small ones) in an operation known as "shot peening."
This is done to hardened ferrous (or similar) parts like transmission gears, and is not for cosmetic purposes,
but rather to close surface imperfections so that they don't develop into cracks.
Use thread-locker inside an engine. Hot oil is slipperier than the sealer is sticky. Never use on rod nuts - it pops right off and clogs oil passages.