I have formulated a strategy for keeping my keywords healthy in Aperture 1.5. I have far fewer images than many people (about 19,000, all JPEGs), and only a couple of libraries to deal with, but my plan should work well for really big collections.

The challenge is this: since Aperture collects keywords whenever it can from where ever it can, any system I set up will become poisoned by imports (images, projects, libraries) that bring in keywords that don't match anything. So I have two things to deal with: recognizing that it has happened, and fixing it when it does. So here is the six K plan:

Klothe, Kapitalize, Kreate, Kollect, Klean, Korrect.

So it's also Korny. But I wanted to used K's if I could.

Klothe
This means that I have no naked keywords at the top level. I other words I use a hierarchy for everything that can be applied, never applying Germany to an image, always applying Geography > Europe > Germany instead. This makes it easy to see if any new keywords appear at the top level. There should be only ten or so top level categories.

Kapitalize
All my keywords will be capitalized. I could prefix them to make extra sure that nothing unusual has snuck in: instead of Groom, use bgtGroom for instance. But that looks like a lot of trouble for little gain. Besides, capitalization looks right and is easy to do.

Kreate
Create the keyword system I want and implement it. Do this first. This gives me the ability to rekeyword whatever I find amiss. For example I can change anything with the naked keyword New York to Geography > United States > New York > New York City. At the top level I will have a category: Geography, Content, Actions, Event, Location, etc. and beneath that have a hierarchy of keywords I can use, applying only the lowest level keywords to images. The reason for the categories at the top is that I can have a keyword like Vermont and store its meaning too: Content > Food > Cheese > Vermont is not the same as Geography > United States > Vermont, for instance. I could have a photo of Vermont cheese taken in Hawaii. The benefit of this organization will be greater once Apple fixes Aperture so that the hierarchies are better displayed and made more usable for filtering.

Kollect
Collect all the keywords everywhere in all my images and libraries. Do this second. This is an important step. By opening all my libraries one at a time, Aperture will collect all the keywords from each of them (and update them to 1.5 format if needed). So when I am done I will have a list of all the keywords there are. From here on out there will be no surprises.

Klean
Clean up all libraries. Do this third. Go through each library one at a time and delete any unused keywords and rekeyword anything that does not fit my planned scheme. There will be no keywords left at the top level once this is complete. As each library is opened, old keywords will (re)appear, but once the last one has been processed, everything will be under control.

Korrect
Every so often, new keywords will appear. They should be easy to spot and easy to deal with. If want to be really careful I can export the keyword list every so often and compare it to a master copy that I keep outside Aperture.

Keeping the Keyword HUD locked is a big help. Not only does it stop keywords from being moved or modified accidently, it also keeps any keywords that come in with new images (imports) separated. They all go into an <Imported Keyword> section at the end of the top level list. Ideally, I think the Keyword HUD should be locked automatically each time Aperture is launched.