I'm in the process of keywording my entire LR library at the moment. It's been ongoing for a couple of years now as it's not the most fun job in the world, but I found a method that allows you to tackle it piecemeal which makes the tedium somewhat manageable.
Finding photos can be a royal pain - especially ones of family members, but keywording removes that issue entirely, allowing you to do a quick filter for all photos containing one or more keywords. It's also useful if you take stock photographs as potential buyers will be using search terms to find your image.
To begin with, I created a keyword set with 9 photographic "styles" to break down all my photos into manageable chunks. For example, Abstract, Animal, Architecture, Landscape, Nature, Portrait & Still Life. This left me with 2 "spare" for styles I shoot the most - in my case Street Photography and... well... I called it "Gothic". I'm a "tidy" person, so these "styles" were all grouped under 1 keyword heading.
Generally, I always go out and shoot the same type of "stuff", so my photos were all organised into groups of similar images. A day out usually resulted in landscape photos so they could be grouped up with a single SHIFT+Click and given the "Landscape Photography" keyword. Giving all my photos just this one keyword was about the worst of it. From there on in, I used Smart Collections to collect my work into photos that needed keywords, were finished, and were partially finished.
My Smart Collections are set up like this...
First - I have a "Keywording To Do" collection set. This contains a Smart Collection set to pull out all photos that have the keyword "Abstract Photography" (for example) and are NOT labelled Green.
I also have a _No Keywords (underscore puts it at the top of the list) which pulls out photos with no keywords at all. This usually populated when I import new photos into my catalogue and tells me that I need to *at least* add a style keyword from my keyword set. Finally, I have a "keyword complete" collection which finds all photos with a green label.
As you can see, I use the green label to identify photos that have all their keywords in place.
What this does, is gives me 9 smaller groups of photos that I can work through at my leisure. Once an image is keyworded, I turn it green and it disappears from the collection. As you can see from the screenshot, I have done 3800 images, but still have Architecture, Landscape & Still Life to complete.
Adding keywords is dead simple for me. I write one sentence in my head that describes the picture and enter three keywords that are important in that sentence. These words can then be dropped into categories shown in my keyword list screenshot above.
Actions: Tend to be anything ending in "ing" (running, smiling, laughing etc)
Animals: Dog, cat etc, etc
Colours: Predominant colours in a scene
Concept: this is for concepts like "love" or "humour" or "irony"
Effects: Panoramas, black & white, infra-red, long exposure, light painting. This allows me to find similar images easily.
Event: Holidays, Weddings, and other big events are given a keyword. I don't shoot for stock, but I do like to find my images, and being able to type in "USA Road Trip" and find all images associated with that is useful.
Family Favourite: These are family photos that might end up in a book someday.
Land: Trees, rivers, mountains
People: This contains all my names of people. I tend to go with first name + surname initial as this is enough to differentiate people. Again this is used to I can find all photos of "Ian T" for example.
Style: This contains all my "Landscape", "Portrait" keywords.
Places: Location & country names.
Structures: Houses, churches, bridges. Anything man-made.
Transport: Planes, trains & automobiles (these could go in "object" but I wanted them separate).
Utility: This is my container for all keywords that are useful but don't necessarily describe the content of the photo. "Neg-Scan", "Contact Sheet", "Screenshot", "Video", "Rejected" etc.
Once the keywording is complete, one thing I do is save the metadata. This copies the metadata changes back to the original files, effectively embedding the keyword data in the original image. This means if I ever lose my catalogue, or if I move the pictures somewhere where I don't have Lightroom, I will still have the keywords embedded in the image file.
This is also useful for rejected images. I'm a hoarder, and disk space is cheap, so rather than delete from disk, some images get a "rejected" keyword. Once this is saved back to the original file, I can delete the image from LR (but *not* the disk) and know that if I ever reimport it, it will have a "rejected" keyword reminding me why it's not in LR.
Hope this helps!