saving files to folder where files came from?

Hi, newbie here. Macbook purchased in december 2008 with latest OS X 10.5.6. Was wondering if someone could help me figure out how to automatically save files I have been working on to the folder from which it came and not the last folder which was opened? For example, i am working on an excel file opened from Folder A, then I continue to work on other files from folders B, C, D. It is frustrating to use the "save as" feature for subsequent revisions and have the file from Folder A get put into the last opened folder, Folder D perhaps. I'm sure there's an easy way to do this but haven't a clue how to even ask the question..... Hopefully there is a way to set this up. Thanks and if you'd like to email me directly, my email is [email protected] Cheers, Eric