I usually take the lazy approaches.

[u]Maps: Google Earth[/u]
[b]Pros:[/b] Fully interactive, can readily designate precise locations, zoom function, street-view, can link to websites of local businesses and buildings to figure out what that place is about well enough to reasonably contact the owners and get permission.

[b]Cons:[/b] Bandwidth-hog, it's google, only does RL locations, can get lost (yes, on a map, using a map), and some people don't like the idea of you playing 'pretend' using their property as a basis for anything.
([url=http://tvtropes.org/pmwiki/pmwiki.php/Main/StreisandEffect]upside is, at least you aren't LARPing on their property[/url])

[u]Existing images:[/u]
Pros: readily available, minimal art skill required, time-saver for common objects in need of extreme detail
-May also work for strict fandoms

Con: Can spend a lot of time cruising for the "perfect" image, generally not interactive.
-When working with strict fandoms, [url=http://tvtropes.org/pmwiki/pmwiki.php/Main/FanworkBan]this may happen[/url].

[u]Composite of existing images:[/u]
Pros: allows players to use readily available images to mix and match the appearance of things using that thing called imagination, allows for customization.
Con: Time spent looking (although not as bad) and requires imagination, not interactive.

[u]User Modified/annotated images:[/u] Includes template-drawings/images.
Pros: allows you to have fun with google-maps and make your own maps with minimal effort
Cons: Not very interactive (interactive being, to tell the editor to re-mod and clarify a subject, which is time-consuming)

[u]Freehand drawings:[/u]
Pro: You get it exactly how you want it, only have yourself to blame if it isn't right.
Con: [i]HUGE[/i] learning curve.