All in the setting
One of the things that really helps with the atmosphere of any RPG is having the setting down right. In the fantasy based games I run that is fairly easy to accomplish as they are not required to stick to any hard laws of anything, including physics. Magic, deities, or mystical creatures can go a long way towards explaining away any difference between your game world and the standard reality that we all live in.
However, when you are running a modern or near future game the requirement for the setting to more closely match the reality we live in increases greatly. One way to add that finishing touch is to have detailed maps of the major buildings that are part of your plot. With all the detail that goes into a modern building this can be a major time sink for the DM. To help solve this problem, here are a number of links to various building types whose floor plans are out on the internet. If you don’t find what you are looking for here, just do a web search for building floor plans.
Floor Plan Links
Science Research Building (Stanford’s Lorry I. Lokey Stem Cell Research Building)
Academic Building (The Centennial Campaign for UW-La Crosse)
Observatory (Griffith Observatory)
Office Building (Wellington E. Webb Municipal Office Building
Auquarium/Oceanarium (Shedd Aquarium and Oceanarium)