It's a good idea to add your city name to your site pages as well. Having the ZIP appear on your page may be good for local search engines if they associate that zip with cities that users are searching upon, but most users are likely searching with the City Name instead of ZIP, so you need the city name for more precise term matching potential. If you operate out of a street address, it should be also added.
I recommend using the hCard microformat for including your local information on your webpages, though that's not strictly necessary.
In addition, you may want to expressly register with online directories and local search sites instead of just expecting the usual web spidering to pick up your site and include it in local search/directory sites. If your listing isn't shown in Google Maps, Yahoo Local, and top online yellow pages, you should go into them and submit your information. Even if you're already listed, it's maybe a good idea to register so that you can add to your business information they have on file, and update or correct anything shown about you. Submitting your info can give more normalized info about your company/site, and reassure the engines that your info is correct.
I also recommend listing your city locality in the city Title tag, and make sure the META description tag mentions your city in the really brief number of words that are likely to show up on search results pages (say, the first 25 words). You should also mention the type of business you are in addition to your name and locality. Ex:
"Ajax Photography Studio - a photographer for the Seattle WA area"
Finally, some of the local search engines use a combination of factors for which businesses might appear first in the search results. If someone is searching for "Plumbers" in New York City, there are many thousands. Those plumbers who have listing addresses closest to the city's centroid may appear first. Of those nearest the centroid, those which have higher user ratings in various sites might be ranked highest.
So, it's also helpful/important to work on getting your happy customers to go and rate you in various online yellow pages, in local search engines, and in speciality vertical sites catering to your type of business.