Of late I have become very interested in tagging in all sorts of forms. I've been extensively tagging my e-mail and also images and various bookmarks (using sites such as Delicious). Here, I would like to explain a little bit of my personal philosophy of tagging.
In general, the way that I've tried to organize much of my information is not to have individual pages (or items) that are grouped into sub-catagories but rather to think about all my activity as a variety of ongoing "streams" -- for instance a stream of links, a stream of images, a stream of papers -- and then to have different tags that can be associated with these different information streams, which relate different entities and different streams together and also to allow one to follow a group of entries in one stream over time.
Consequently, it is very important to have tags that are unique over all these information resources (e.g. across bookmarks and emails) and so can be readily searched. This is the logic behind my letter and number tagging system. That is, I find the best way of making up a unique tag is by associating a particular word with some numbers, for instance something like "pod57". Many types of searches (e.g. with gmail) do not include punctuation characters such as – (dash) or _ (underscore) or certainly space. Furthermore, not having numbers often makes the tag word not unique and not that discriminative. For instance, the word "pod" itself would not work, but if one conjoins with a digit (e.g. "pod57"), it becomes unique.
In addition to creating unique ideas that link entries between various information resources I also use tags to circumscribe the meaning to some degree of concepts. For this I just use normal English language words and then look at the union of all words associated with an entity.
Finally I also have developed a tag hierarchy sometimes where I will have an overview term (e.g. "clip") and then a sub term (e.g. "clipblogpost") where one can clearly see how the secondary term is a sub term of the first.
With this I hope all of this cryptic tagging is a bit more comprehensible.