1) don't use the official client.
2)you may actually have to set a limit on your upload speed. I know with my current connection that if I set a max upload speed of greater than 17kb/s, my download goes right down the shitter. I don't know why, but I could be downloading at 100k/s and uploading at 13k/s one second, but as soon at it hits 18-20k/s, my download speed drops down to like 1k/s.
3) fiddle around with the max number of connections on a file. if it's too low, you won't be able to DL from enough people to get a fast Dl going. If it's too high, it goes screwy.