One of Dr. Phil McGraw's Life Laws is that we teach people how to treat us. Some of us seek to please others so much that we end up teaching them to have little or no regard for us and thus end up being treated like a doormat. However just as we taught them how to treat us poorly, we can also teach them to treat us better. It all starts with recognising the signs of this troubling behaviour.
Lack of Boundaries
One of the first ways to tell if you are being treated like a doormat is noting when your friends, family or lovers do not recognise or respect your boundaries. Perhaps you have neglected to set any by refusing to say no to any request. The way to reset this unfortunate circumstance is to reclaim your "no." Learn that it is not only OK to say no to a request, but also necessary.
Lack of Reciprocity
Relationships exist successfully with the balance of give and take. If you find that you are always expected to give and you get very little, if anything, in return, this is a glaring example of being treated like a doormat. Relationships should never work as tit for tat, where you keep a vigilant eye on some emotional or physical scorecard. However if you are constantly drained by your relationships rather than renewed, it is time for a re-examination and renegotiation.
Lack of Respect
The lack of boundaries and the lack of reciprocity both stem from a lack of respect. This goes hand in hand with self-respect. Someone who has a healthy self-respect would never allow themselves to be treated like a doormat. The easiest way to stop being a doormat, therefore, is to learn how to respect yourself. Learn that you do have something to offer and you don't have to barter for your friendships out of your own precious esteem.