I generally focus on a core idea in my hub, find one or two ideas to create a basic keyword list.
So say a hub on the Kindle Reader, this basic list would include; amazon, kindle, e-readers, nook.
I would then publish the hub with that small core of keywords or tags.
After the initial rush of hubbers finding the hub, generally after one or two months the keyword statistics would begin to come into focus on the hubs statistics menue. I then expand the keywords to include some of the more highly rated suggestions by the program that finds links to the previous 30 days search terms sent to search engines.
You can always go to Google AdWords too at the very start to see what keywords are being chased by advertisers, but personally I have found this method lends itself to keyword stuffing.
Another trick is to read other hubbers work and play a game. You are a Search Engine, now find as many keywords in the article as you can. Some hubbers who stuff are easy, the really good ones are the hubbers who make it seem they have no keywords.