Teaching children it's ok to express anger

[Simic]

I have read quite a bit recently on how it's important for children to learn that it's ok to express anger and that children learning it is not allowed and so having to suppress the angry feelings can lead to problems. Does anyone have any ideas (or book suggestions) as to how to do this? I think I myself have trouble expressing anger - at least to anyone except my DCs and my sister :(
How can I teach my children to be different? I don't think the answer is for me to shout at them more! :o

I told my 4 year old to go into mummys room and just punch the pillows.

I was surprised by the amount of punching and for how long .

Ive done this twice and he loved it and came out smiling.

Id actually forgotten about it so thanks for reminding me!

I should add - not just randomly - when he was getting angry with me or his little sister.

my 5 yo dd writes it down sometimes if she is angry with/ about her school friends (Id forgotten about that too-so thanks as well). she had a little book for a while. Lots and lots of 5 year old expletives but it really seemed to get it out of her system