my principle right now is to tell the truth wherever possible. if i feel i can't, or am going to hurt someone where i should not, then shut up. (there are situations where one does have to mention unpleasant facts because it is going to be more useful in the long term, those are different)
i used to lie a lot as a defensive mechanism. i was afraid of someone overreacting to something i would say or do and then myself overreacting to them and feeling scarred, so i would hide, evade and tell outright lies. i've grown to be very good at that. i think this is the reason why i am so sick of it. i would rather not have to do this again if it can be helped.