Good question. The 'ruler' of a planet is called it's 'dispositors' in western astrology. So if your Sun is in Aries, for instance, it's dispositor is Mars.
In Vedic astrology in particular, they assign a lot of importance to a planet's dispositor . A badly placed planet might otherwise be strong if it's dispositor is well placed, since the dispositor of a planet acts like it's foundation / support.
This also allows you to look for dynamics such as the "Mutual Reception", where a planet's dispositor links back to it. For instance, if your Sun is in Aries then it's dispositor is Mars. And if Mars is in Leo then it's dispositor is the Sun, so it links back to it and they continuously share energy and support. In this example, they're said to be in Mutual Reception.
Also, by looking at the dispositor of each planet in your chart, you can look at the roots and can gain further insight into the dynamics of your chart in this way. The root of a dispositor chain is called the "Final Dispositor ".
So for instance, if your Sun is in Aries, Mars is in Virgo and Mercury is in Gemini then your Final Dispositor for this chain is Mercury because the dispositor of the Sun is Mars and the depositor of Mars is Mercury and Mercury has no depositor because it's in it's own sign.
In my case, all my planets eventually link to Mars through following the dispositors down until you can't go any further. So Mars is the Final Depositor of my whole chart. For others, they may have several Final Depositors as roots in their chart or the chain may end with a Mutual Reception.
*ed* Here's a good article on dispositors
https://web.archive.org/web/20050711160923/http://www.panplanet.com/library/dispositors.htm