http://www.thonky.com/eve-online-guide/corporation-standing.phpWell i was confused as my standings didn't increase as I expected so I decided to find how the heck it works.
There are apparently 4 things that determine how fast you gain standings with a particular corp.
1. agent standing increase=base mission standings increase + (1 + (0.05 * social skill level))
2. corporation standings increase = agent standing increase* agent efficiency/corporation size
3. Agent efficiency= 0.01*((8*agent's level) + (0.1* agent's quality) - 4)
4. New standing = old standing + ((10 - old standing)*corporation standings increase)
Corporation size is a number between 0 and 1, so larger corps are harder to gain standings with.
Base mission standings increase is something each mission has.
So to gain standings look for the smallest corp you can find and do missions for them
edit:
So it should always be better to move to a higher level agent regardless of quality.