The best way to do this is to make sure, that you have a postal adress (i.e. an office) in all cities. If you do, create local business pages on you domain:
domain.tld/computer-fixing-new-york.html
domain.tld/computer-fixing-san-francisco.html
Make a little page about your office and your services, use schema.org to mark the adresses (-> http://schema.org/LocalBusiness ) Use pictures etc.
Next: create corresponding Business Profiles with google and link to your local pages.
Get local links and reviews for your corresponding pages.
There are business related keywords that google doesn't recognize as local businesses. These are usually very easy to rank for (very few, but there are some): For those phrases a simple optimized page "Keyword New York" etc. will suffice (use the seomoz "perfect cupcake" as an example. To test whether a keyword is considered local just see if there are the 7 or 10 local results in the SERP. If not, you might have found one.
