Thank you everyone for the advice, I shall try to follow it all!
Uwe: yes, the download rate is damn good, considering it's only ADSL. I've made sure to force encrypted connections. I had the upload rate set to max, now I've set it to something smaller manually so it doesn't choke.
However I can 't find the integrated upload speed test you mentioned; perhaps it's only available on the Windows client?
Many thanks all; if I can't get my ratio up quickly, then I'll be stuck in upload-only mode for a while. C'est la vie, and it's fair. I'll just have to ask people to download stuff I've got!