Ways to impact public policy. In what situations are different options better? Are any better or worse in general?

For any cause area there are many different different ways to impact outcomes via public policy. 

There are many ways to impact public policy:
- national Civil Service
- subject matter expert
-  think tank
- policy at a company 
- consulting

And there are many routes through these different roles.

Do you have any thoughts as to which routes are better or worse or on what factors one should weigh them?

If you did one of these, in what kinds of people/situations would you recommend it and in which would you not?