In the process of creating targeted keyword lists for any particular project it's easy to simply say "target the keywords with the highest search volume". This is almost never true, of course. Clients often want the moon delivered on a platter but that doesn't mean that's what they actually need. The following tips are a few of the tricks I've learned as I cull a keyword list.
Using Plural when Singular is an Option
This goes hand in hand with realizing what the intentions of your audience are. If your potential customers are searching for a birthday present for their mother, they are not going to look for "PS3 controllers" when "PS3 controller" will do. They're only looking for one.
Google Trends shows this effect:
There's nothing wrong with talking about the product in the plural on the page. That should be all the targeting you need for those terms.
Side note: your customers are going to be in trouble for giving a PS3 controller to their mothers.
Targeting Local Keywords when You're not Local
If you have an international market, then trying to be in every local search becomes painful. Target the global terms if you're a global company and let the utilities mentioned above do the work for you to get in the local markets.
Actively trying to target a misspelling is terrible waste of time. The SERPs are going to realize the mistake and send your customers to the correct spelling anyway, negating any work you may have done.
The current suggested course of action is to include the misspellings in the meta keywords. While it's already known that Google does not use the keywords tag for any of their weighting algorithm, it is suspected that the keywords are used for relevancy -- a perfect place for misspellings.
Side note: "misspellings" is very easy to misspell.