scratching your back?

Nothing scratches your back quite like your fingernails, 

So take upon yourself all of your affairs. 

And if you are in need of something, 

Then seek it with one who knows your worth.

This poem uses the analogy of how one who wants to scratch his back but uses someone else to do it for him, will never be able to perfectly pinpoint the itch to the person doing the scratching, however good his instructions are. The best scratch possible for him is only by his own hands. The Imam is advising us using this analogy to become self sufficient, as that’s better for us and as it means one isn’t encumbered by favours from others.

However if one really must get help, then the poem advises to get advice from one who knows your worth, or from one who won’t exploit the fact that he’s helped you. This is the one who knows that you too have skills and things he might later get help in from you, or he respects you for whatever reason and helps you due to that respect, rather than expecting any reward from it afterwards.