When should 之 be used as a particle? There doesn't seem to be a straightforward answer on Nciku. On Wiktionary, the entry states:
Indicates that the previous word has possession of the next one. In English it functions like 's or like the word "of" but with the position of possessor and possessee switched. 的 (de) is more common but 之 is used in many set expressions.
However, in the contexts I've seen so far, the 之 particle is used more like 的 in some sense (rather like the Japanese の perhaps) - for isntance, 银之匙 as a translation of a manga title - so the usage seems to be more of a "link" between two different words as some sort of descriptor or identifier (in terms of a quality of the second word, in a sense).
That said, the purpose of the 之 in 之前 or 之後 seems to be slightly different than in the other example I gave. Taking all this into account, when should I should 之, outside of expressions that have it already (such as 之前), and what makes in different from 的 in phrases such as "银之匙"?