by using software like Laplink
click here. This enables you to link to another machine via the internet, or by dialling directly into the other machine - which involves the cost of a phone call while you're connected.
Another way to share files, and one which a lot of people use, is to do it via private web space. It's easy enough to set up a system that will enable you both to transfer files to the site via FTP, or if you want something more ambitious, via a 'proper'web site. I wouldn't recommend this method if the files involved are in any way sensitive or confidential, but the big advantage is that you can acces the files at times when the other machine is turned off.
It's a subject that could take up pages, so I'll leave it at that - if you need more help just ask.