Professionalism is difficult to define with formal rules, and it is even more difficult to practice. The essence of professionalism, however, is an attitude of caring about the interests of the clients you are called upon to serve. This attitude will cause you to hold yourself to high standards and will eventually benefit your career (and your self-respect) more than any ethically dubious business practice ever will.
Obtain a copy of the professional code applicable to your profession and learn it by heart. For example, if you are a lawyer, then obtain a copy of your state bar association's rules of professional conduct. If your profession has not established formal professional standards, then search the Internet for informal standards. As a last resort, create your own professional standards applicable to your profession and write them down.
Create a set of written personal goals designed to improve your professionalism. These goals should include ethics, appearance, expertise and time management.
Become socially active in your professional association. Identify true professionals and seek to befriend them for mutual encouragement and support. Remember that true professionals are often quiet and unassuming -- they are not always the people who seek to have their names associated with ethics committees and such.
Read books and attend seminars on professionalism. These resources will not only teach you how to practice professionalism, but will also inspire you to consistently practice what you already know.
Take night or weekend classes in your field in order to improve your knowledge and skills. The sense of achievement that you will gain can encourage you to take pride in the quality of your work.
Create a personal journal in which you list your successes in the development of professionalism. Review this journal regularly. Try to be positive -- if you made a mistake but made amends rather than excuses, record it in your journal and count it as a net gain.