Always write in pen.
You might want to write in pencil to make erasures and keep your notebook neat.
However, many schools require you to hand in a carbon copy before leaving the lab, so you can’t erase anything anyway.
Always record data directly into your lab notebook, not on scraps of paper for later transcription (what would happen if you lost the scraps beforehand?).
If you make a mistake, draw a single line through the observation, with a brief notation why you are crossing it out.
Never scratch something out completely so it becoes illegible (you may later want to know what it was or why you crossed something out).