Here's a few:
1) When using a crank arm puller, don't engage the puller until the threaded part is fully engaged on the crank arm threads
2) Don't spray lube directly on your cassette or freewheel, its better to leave it clean and dry.
3) Don't tension your spokes with the tire and tube mounted.
4) Don't overtighten quick release skewers.
5) Don't overtighten crank bolts and don't grease the tapers.
6) Don't raise your stem or seatpost above the maximum line.
7) Don't panic when mounting a tight clincher bead, often it slides right on after a few trial runs.
8) Don't trim new cables too short, often you'll need the extra length for your 4th hand tool, Do use cable end caps.
9) Don't fail to grease your (old school) alloy seatpost or quill stem.
10) Don't take every bit of play out of your hubs when repacking bearings. They should be left with a slight amount of play so that when secured to the frame with the quick release skewer, the tension will snug up the bearings that last little bit.