In an ideal world, you'd fit one of those filtered faceplates on your master socket, and plug the modem in there. The faceplate provides an ADSL socket, and the ability to filter the ADSL signal out for every other phone socket. That way, the ADSL signal isn't going all around the house and picking up noise.
From what you've said, that isn't possible as you want/need the modem upstairs.
That said, as Burakkucat says, make sure the ring wire is disconnected at each socket, that can make a distinct improvement.
I'd also temporarily try removing the lower front panel from your master socket, and plugging the modem in there. Note the stats from the modem (synch speed, attenuation, S/N margin etc) in it's current setup, and when temporarily plugged into the master socket with everything else disconnected.
That way, you'll get an idea of the magnitude of improvement from replacing the front plate and possibly doing a bit of rewiring.
It may be that the improvement is huge, and thus should spur you on to do the work
Or it may be such a slight improvement that you decide it's not worth the hassle.
(I'm assuming here that your master socket resembles one of these
http://www.kitz.co.uk/adsl/socket.htmin which case, follow the instructions on the top of that page to see what potential improvements might be made)
Ian