43 Folders has written a guide devoted to the fine art of tagging. No matter what the data is that you're tagging - blog posts, photos on Flickr, or bookmarks at del.icio.us, a good tagging system will help you find what you seek with ease. This three point guide basically says consider what you're tagging and how you'll remember them (ie photos are often associated with the place they were taken, or the person they depict). It also points out you should consider what needs to be tagged - for example, OSX files already have the date created and modified attached to them, so tagging them with a date is redundant.
FInally and most importantly, create a list of attributes from which you'll draw your tags.
"Write out a list of the attributes that you think of when thinking of your target items. Ideally, you should make this a brainstormed list that includes every attribute you can possibly think of that you might want to tag. As you make the list for your different target items, star the attributes that spring immediately to mind.
Once you have a list, go through it to weed out the attributes that are covered by the item’s non-tag metadata. Then go through it again and pick out what attributes you want to use for tagging. Try to keep it a short, specific list focused on the attributes that sprang immediately to mind. You should also add attributes that didn’t spring immediately to mind, but that you want to make a habit of tagging anyway because they will be useful."
Having this list available (printed or in digital form) will help you remember the tags you want to use until you've developed good tagging habits. After all, it's much easier to tag items correctly the first time rather than having to go back and fix them up in a batch later. Trust me, I've been blogging since 2000 and only started tagging this year (sob).
Becoming a tagging kung-fu master [43 Folders]