In my career I have estimated a lot of project. I tried different techniques and I find https://en.wikipedia.org/wiki/Program_evaluation_and_review_... is the most accurate one.

If I was asked to give some hints, I would say:

- in optimistic and pessimistic cases don't fear to use extreme values

- don't estimate on your own, take reasonable amount of developers with variety of experiences

- don't give exact value to client, give probabilities ("it is very likely that we will be deliver this in between X and Y man-hours/days)

All estimates I made using PERT were quite accurate (<5% time difference between estimated and real execution)