FrontPage RPC is the way to go for uploading and downloading documents from a SharePoint document library from remote clients. But many people find it too complicated to start with, basically also because there is not a lot of documentation on it.
Erika Ehrli has a good example of how you can create
your own Web Service wrapping up some object model code to process the uploaded file and then deploy that Web service nicely in the SharePoint environment. Definitely a good example but do know that it FP RPC is the more performant and scalable way of doing this kind of job.