Don't make assumptions

This is probably the most important tip on this website.

Assumptions can ruin any family tree .

Concidences do happen and the fact that you are looking for John Smith born in 1816 at East Ham does not mean that the John Smith born in 1816 at East Ham that you have traced in the census is your John Smith.

Double check other details like wifes name, home address etc and if necessary back check the person you have found.

By all means list John Smith as possible and even consider him probable but do not list definite until you can be sure.

This is the most common and most serious mistake made by inexperienced researchers and is an error that most professional researchers will have learnt the hard way at some time in the past.