Hmmm, well I have no children yet but we always tell our 18 month(ish) year old nephew "be nice".  Not that he's mean, but that's our way of telling him not to be rough if he starts pulling on the blinds or something :)  I think it's pretty good advice ;)

The best advice I ever received was advice I already knew, but the fact that people told me about it (over and over) helped to reinforce it.  That advice was to save money at a young age!  I had so many friends that were living check to check throughout college and still are b/c they had to have their fun.  I didn't make much money and I saved almost every dime I made.  I didn't go on random vacations and because I saved, my husband and I were able to go on 4 vacaions since we got married AND we didn't even have to stay in cheap motels!  I am still very frugal, even on vacation, but learning to save money "for a rainy day" has really helped.  Now I try to make sure I have x amount of dollars in the bank all the time in case (god forbid!) we lose our jobs.  In this crazy economy, anyone without a savings accont is super stressed.  I'm still stressed, but not SUPER stressed. 

Also, at work they keep trying to reinforce that "young people" should start investing in their 401k's. Well lucky I've been putting a little in my 401k since my last job b/c one of the lawyers told me he wished he would have started saving for retirement at my age.

Ok here's one more - the best advice from a book is this one:

You can be happy no matter what!

If anyone is stressed about anything or even just in a bad mood sometimes, you should read this (I reviewed it if you want the cliff notes version) :)  I try to pass that advice from that book on to other people b/c I really believe it!