Have you read "The No Cry Separation Anxiety Solution" by Elizabeth Pantley? I used her No Cry Sleep Solution book for toddlers and it actually worked to get my son to sleep through the night. So I am a big fan of hers. I recently checked out the Separation Anxiety book from the library but I haven't read the whole thing yet so I can't comment really if it works. She does say that separation anxiety is completely normal and evidence of the strong bond your baby has with you.
One thing that stood out for me was her recommendation not to hold the baby in your arms and pass him to whoever you are having watching him as this frames that person to your baby as the person who is taking him (her?) away from mommy. Rather, have him play on the floor and when you leave say goodbye to him. If the baby gets upset that person can pick him up and be the rescuer.
What you are doing, the small periods of time without you are good practice I think. For me, even just letting my son play independently was something I needed to start doing. I always want to be talking to him and explaining the world to him.

And when there are visitors I try to let them play with him without me. I started going up stairs more frequently to get things without him and tell him, " Mommy will be right back!"
I really think it is more of a developmental thing than anything I have done, BUT suddenly my son says "Hi!" to everyone and even plays peek-a-boo with random people like the lady at the bank without solicitation. I thought he was shy.
I think it just boils down to practicing separation in manageable chunks for your little one. Then staying calm yourself and not making a big deal about the separation. Babies pick up on your feelings.