Bongofish

Here's a quick one I picked up in a mechanics mag ages ago, it's been surprisingly useful down the years.

When you're shortening a bolt, find a nut of the right size, and bolt it right down before you start with the hacksaw.
Always use a vice, (extra important if you want to use a power tool.

Once you've made the cut and filed it off nice and flat undoing the nut will smarten up the thread where it's been wrecked at the end. 

Wow I doubt I would have thought of that, am loving this tutorials section!  ;D