With iStock, only the keywords count for searching within, the title counts for SEO, but not for internal searching and the description not at all.

For your own sake, try to keyword as generally as possible in what's embedded in your file's IPTC data and you should always include all relevant keywords, regardless of whether they're in the title or description.

Stay away from specialized vocabularies in your keywords - if it wouldn't fly in a google search, it's language you shouldn't be using (and I'm thinking here particularly of Getty/iStock controlled vocabulary; DeepMeta can help you disambiguate and translate keywords to CV)

Don't shorten or lengthen titles/descriptions based on any agency rules - these files will probably have a longer life than your tenure at some of the agencies and you want the right information to be in there. You can edit down (or pad in the case of the daft rules on BigStock and DT that require a minimum length - why isn't "Isolated Granny Smith apple" a reasonable description?) when uploading

Think of SEO when putting titles together - don't duplicate titles and think about how you might type a search phrase if you were looking for that image. I used to do things like with clever or funny (I thought) titles and had a series of images with the same title and am now fixing those as I upload images to my own site.

I've never really had a clear policy for myself on alternate spellings color/colour harbor/harbour or differing suffixes - painted, painting, paint. Shutterstock will remove anything it sees as redundant but for DT (I think) you're better off having singular and plural if you have multiples of an object.

Put your most important keywords first so that if you have to cut because there's a maximum at an agency it's easy to chop from the end (if you use a tool that alphabetizes your entries this won't work, but Photoshop doesn't do that)

Try to remember important variants - yard and garden, for example - for images that are going to be searched for worldwide

If there are plants or birds or animals as the main focus, try and get the latin name or specific names. I found out earlier in the month that I sold a Christmas garland image from my own site because the designer was working on something for a university biology department that worked on avoiding invasive species (in the Pacific Northwest that includes holly) and liked that I had included - and keyworded - kinnikinnick which is native here.

Put in place names and region names for outdoor shots and leave them out for studio stuff. Most agencies will take a dim view if you include multiple locations on the grounds that it could have been there (because places look similar).

I'll stop now It's just that this is a very valuable resource for you and for the long haul; it's worth taking the time to get it right when you do it ('cause it's no fun doing it over again)

I do include both the US spelling the the proper Uk spelling. Although, SS still insist "colour" isn't a word, and I always remind them it is.  Whilst there are a lot of "them" and "us" terms you're pretty safe because "us" non-US people know the US search term and will use that if needed.