[ritwik]

For RA -
What is important that you demostrate the you have RESEARCH CAPABILITY. This can be demostrated by recommendations, research experience and most importantly by papers. Now if you have some publication then you can approach a professor with same taste in research ask him out about RAship and say that you have demonstrated your potential by publishing XYZ in ABC. certain information on your perspective of his current work may be included.

For TA -
Take and score well in TSE. If you have prior teaching experience then that is a BIG plus. But I think TA is generally employed by dept and not individual prof (?) so this may be a little tricky.