How do I maximize my download speed?

Make sure Transmission's port is forwarded. Port forwarding makes it easier for others to connect to you, which therefore increases your speed.

If your router supports NAT-PMP, UPnP, or you have Apple AirPort, Transmission can do this automatically; just tick the checkbox in Preferences → Network.

Make sure you cap your upload speed, so that it isn't flooded. A good rule of thumb is about 60-70% of your maximum upload bandwidth. This can be adjusted in Preferences → Bandwidth, or in real time using the Action menu.

eg. If your upload connection is 256 Kilobits/second, then you should cap it at 21 KB/s (( 256 / 8) * 0.66 = 21 ).

Queue your transfers. Transmission's queue preferences are located in Transfers → Management.

Remember, your download speed is proportional to how fast you upload. If there are many transfers running, then each transfer will only receive a small proportion of your upload bandwidth, reducing their respective download speeds. To avoid spreading your upload too thinly, a good rule of thumb is to have at least 128 Kb/s of upload bandwidth for every torrent you wish to run simultaneously.

eg. If your upload bandwidth is 256 Kb/s, then you should only have two ( 256 /128 = 2 ) downloading transfers in the queue.