Moving between machines is always difficult. If you keep files on your hard drive, problems arise with forgetting files. So, if you copy them to a USB drive, you can end up with two copies to a file, which can be even worse.
One way to combat this is to use an online storage solution, such as Microsoft's SkyDrive. SkyDrive is basically an online USB drive, with access to Microsoft's Office Web Apps. Google has a similar functionality through Google Docs.
However, if you are looking for something that doesn't require an online account, you can also download a tool such as Microsoft SyncToy. This application is free for Windows users, but for whatever reason not installed with Windows when you first install it. If you are someone who has some issues with keeping track of files, this is a highly customizable tool. As opposed to a program like backup, a synchronization program can be set up to work both ways. In other words, if I change a file on my hard drive, it can be synchronized to my USB drive, and if I change a file on my USB drive, it can be synchronized to my hard drive.
For free, it's a great little tool, and a good alternative to accidentally overwriting a file with an older version.
Download SyncToy here