Here you go -
The PS3 Proxy Guide
The PS3 Proxy Server
Simply extract and use the server application.
I do a few things differently. The Url keeps changing if you pause or resume the file or if you start a download while in PSN store and then click on 'Download in Background'. I once landed up downloading Moto GP 13 Demo twice thanks to this once on PC and once on PS3
plus the game was horrible
To avoid that follow the guide till Step 5 and download the file.
For step 6 open the PSN store and head to downloads and click on your file to download. Set it as 'Download in Background' and exit PSN store.
Now clear the logs. Go to Download Management and pause and resume the download. Add the Url you get here for step 7. You will have to do this bit a bit quickly though as the Url can change again on resume. It might take a few attempts as well. To check if the file is being transferred from the PC and not being downloaded again simply check the logs.
If say the path on your PC is C:\PS3\A.pkg and the Url is http:\\
www.abracadabra.com. In logs you will see something similar to -
TimeStamp http:\\
www.abracadabra.com -> C:\PS3\A.pkg
The pointer to the mapped path is important. Simply put - If it ain't pointing it ain't working.
Everytime you bind a new URL the download will start from 0%.
Once the download starts logout of PSN as some games suspend download like GT5, RR7 etc., In short any game that has online feature. Play something till the download completes here.
Enjoy!
P.S.: I hate the emoticons someone bring back the old ones.