TL;DR version: Just use the GPS.
Long version: Turn on Wifi positioning ("Location & Security > Use wireless network"), turn on GPS ("Location & Security > Use wireless network"), use Google Maps (or other GPS applications), then walk/drive around your city.
When you turn on both of these services, before the GPS acquires a "fix", Android will send the Wifi's MAC/SSID and Cell Tower base stations CID/LAC around you to Google's server and Google's server will send back your location information; after the GPS acquires a "fix", your device will send the Wifi MAC/SSID, Cell Tower CID/LAC, and your GPS location to Google's server and Google's server will collect this information to refine their own Wifi database.
note: I believe the device could also be sending other data, like signal strength and possibly ping latency; but I'm not sure about the specifics of these.
Also from Google's support forum:
by Christopher (Google Employee)
If your location is being incorrectly
detected by a Google Maps or Latitude
using Google's cell ID (cell tower) or
WiFi (wireless network) location
database, you can help provide updated
info to correct Google's database
using Google Maps for mobile. At this
time, you cannot provide individual
updates to Google's location
databases, though they are being
updated and improved constantly over
Open Google Maps on an Android
2.0+, Windows Mobile, or Symbian S60 phone and enable GPS. While Maps is
simultaneously connected to a GPS
satellite and a cell tower or WiFi
router, you will be providing updated
anonymous geographic data for the cell
tower or WiFi router to which you're
connected. Please note that this data
is anonymous and may require a
significant amount of data from you
and other users before changes are
made to Google's location database.
Android: You must enable Settings >
Location & security > Use wireless
networks and have previously given
consent for anonymous location data
collection. You can check if you've
given consent by un-checking and
re-checking the 'Use wireless