Like many of you out there, I've searched and searched on the Internet for a solution to this issue, which I've also noticed to be quite popular (why should it be? you create something on the virtual machine and you want to save it, right?), and many guides are incomplete or they point you in the wrong direction. Therefore, I present today a step-by-step tutorial (with screenshots) to this very simple-to-fix issue (you won't believe how simple it was in the end).
On the rare occasion when I have to use Windows I use a VM in VirtualBox, but Windows 7 folder sharing doesn't work as it did with XP for some reason. Easy enough fix--full details in the article but basically you set up the shared folder and then map the drive using \vboxsrvsharename