Getting to assist in the course of your choice during TAship I am an incoming Graduate Student on a TAship. 

I have completed the course in my undergraduation and am currently working on a project in the same area.
I wish to know what else can be done to improving my chances of securing TAship in the preferred course. 

Is it advisable to express one's enthusiasm in the subject with the professor?
It's certainly reasonable to politely express an interest in TA'ing for a particular course or with a particular professor.  You should express that interest as soon as possible to both the professor teaching the course (or the course supervisor in the case of a multi-section course) as well as to whoever has overall responsibility for assigning teaching assistants (typically this is coordinated by some faculty member or an administrative assistant.)  

Realize that the needs of the department and the preferences of the faculty will be prioritized over the desires of graduate students in making these assignments and that you may well be competing with other graduate students for a desirable assignment.  Don't count on anything...