If you want to see what your piece will look like with a clear finish, rub some lacquer thinner on it. This will evaporate off quickly and won't raise the grain like water will. 

You paid for every tooth on the hand saw or file, so use it! You'll get cleaner, faster cuts. 

If you need to thickness a really thin piece of wood, clamp a block to your spindle sander and feed your thin piece through. Then readjust the block for each pass through until you get the desired thickness

When using a chisel to remove wood, take many small, thin shavings, rather than trying to take out big chunks at once. This gives finer control of the final product, is easier to control, and is much easier on your arms!

You almost never need a super long bar clamp. Instead, take two clamps, twist the end 90 degrees and interlock them in the middle, then extend the opposite ends to the edge of your board

If you need to get a straight line on a dowel or cylinder, or need to mark corresponding points on both ends, put the cylinder against a door jamb or wall corner, then use a pencil and draw a line all along it. 

If the water stain is white, it's in the finish, so you can just strip it and refinish. If the water stain is dark, it's in the wood; you need to strip the finish, then use muriatic acid to draw out the stain before refinishing.

If you want your raw wood to look like it's been sitting in the sun for a few months, but don't want to wait that long, put a 4% solution of sodium nitrite (NaNO2) in water on it before the final sanding. This won't harm the wood, it simply accelerates the oxidation process.