If your router supports UPnP or you have Apple Airport, Transmission can do this automatically; just tick the checkbox in Preferences >> Advanced.
eg. If your upload connection is 256 Kilobits/sec, then you should cap it at 21 KB/sec ((256 / 8) * 0.66 = 21).
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 KBit/sec of upload bandwidth for every torrent you wish to run simultaneously.
eg. If your upload bandwidth is 256 KBit/sec, then you should only have two (256/128 = 2) downloading transfers in the queue.