Keyword research - use your Adwords or
Overture campaign to see which are the keywords actually being
used.

Because Adwords and Overture show the number of impressions that a keyword
has achieved as well as the number of clicks, you can then use this
information to decide which keyword to use when it comes to optimising
for your organic results. For example, do more people search on 'jumpers'
or 'pullovers'. Set up a ppc campaign and bid on both terms and see
which gets the most impressions for the same ranking position. This
way you will bypass any possible skew with American and English usage
in the various keyword research tools. Of course you can take this a
stage further and monitor which keywords produce the most conversions!