I have also had personal experiences of these kind of
encounters, as have some of my friends with clients,
friends/acquaintances and even a stranger on a train!
I think its quite common, its just that most people do not
pick up on it or stop to think that another person could be
responsible for them suddenly feeling 'Drained'.
A friend of mine who is an EFT meridians practitioner and
reflexologist stopped doing phone consultations for a time
because of a traumatic 'Energy Vampire' experience. (She
has since used ET on the experience and is happily taking
calls again!)
As someone else mentioned, I think the visualization method
is a good thing to try.
Visualize a protective white light around you- an energy
shield. You can still interact with the person on all
levels, receive what you need to receive and give what you
need to give- but you protect your own energy levels.
Take a deep centering breath and follow it- telling yourself
that you know what is happening and you can deal with it.
Keep your personal power, particularly if your with a client
when this happens.
And don't hate them! I really don't think most people are
aware that they do it- most people aren't that clever LOL
Although it dose make you wonder why there own energy source
is so low or why they need so much…