should I reindex the table of more than 10 millions rows regularly when more data are inserted?

    I have a big table (more than 10 millions rows) and create an index (three columns) ( the index is not created with creating the table).  Within one day of the index created, the performance of query searching is ok.  But with more than several thousands rows of data  inserted into the table, query searching becomes slower and slower.  

   Should I recreate the index regularly when more data are inserted into the table? 

Thanks for your help in advance.

ouyang