Shavings can pile up around a hole being drilled, making it hard for you to see what you are doing. To prevent this, push the point of the drill bit through a 4-in. strip of masking tape, then draw the tape up the bit so it will clear the work. Fold the strip lengthwise to bring the sticky sides together. The tape "wings" will act as a fan to clear the surface.
Attic work is hot and tiring. Work in the morning before it gets hot and keep lots of cool liquids handy to avoid over heating.
Be especially cautious if a contractor: comes door-to-door or seeks you out; just happens to have material left over from a recent job; tells you your job will be a "demonstration;" offers you discounts for finding him/her other customers; quotes a price that's out of line with other estimates; pressures you for an immediate decision; offers exceptionally long guarantees; can only be reached by leaving messages with an answering service;
When measuring, the old adage "measure twice and cut once" can be excellent advice. Pros, however, have their own variation; they usually only measure once, but then will read the measurement twice. They will mark the cut line, then cut the wood just so the saw kerf is touching the line.