Keywording 101: Stick to your basic IPTC core metadata procedure then adapt it to which ever site and their quirks may be. Green, Tomato, Tomatoes, Green Tomato, Green Tomatoes...
As mentioned above, various sites seem to do what works best for them and vary greatly in how to actually upload the data. Some sites actually require you to use British English and American English, which is a real pain in the you know what... harbor, harbour.
Also of note, if the photo has one green tomato, the don't use the keyword tomatoes, and if it is a pile of tomatoes, don't use tomato.
And lastly, accuracy, accuracy accuracy. I can't stress this enough. Think like a buyer, and if you want a green tomato and that is what you searched for, you don't wan't to find one red tomatoes. Keyword spam hurts us all.
Also another deal breaker is if there is a tomato in the shot, but it's not what the shot is about, then don't use tomato.
Apart from the the 5 W's - Who, What, Why, When, Where.
I rarely use more than 15-20 keywords and only use more if it really requires it.