Limit your upload speed to approximately 80 percent of your maximum upload rate. You can check your upload speed over here (never trust your isp). Once you know your maximum upload speed, change the max upload (to 80%) speed in your torrent client's preferences.

If you're on XP sp2, your TCP connections are limited to a maximum of 10. This seriously hurts your downloading speed because it wont let you connect to a high amount of ip numbers. It is supposed to slow down viruses because their spreading strategy is to connect to a high amount of ip numbers, but it also cripples your torrent downloads.
A nice way to fix this is to download this patch, it allows you to set the maximum allowed connections to any number you want. Any number between 50 and 100 is ok (I put 512 though.) (more on this).

A simple tip, but o so important. Always look for torrents with the best seed/peer ratio. The more seeds (compared to peers) the better (in general). So 50 seeds and 50 peers is better than 500 seeds and 1000 peers. So, be selective.

By default, BitTorrent uses a port 6881-6999. BitTorrent accounts for a lot of the total internet traffic (1/3), so isp's like to limit the connection offered on the these ports. So, you should change these to another range. Good clients allow you to do this, just choose anything you like. If you're behind a router, make sure you have the ports forwarded or UPnP enabled.

It sucks. Windows Firewall hates P2P and often leads a life of it's own. So disable it and get yourself a decent firewall, Kerio or Zone Alarm for example.

The default setting in Windows XP for maximum connections is 8 which is not optimal enough for torrents to connect to seeds and peers. The best way to increase the number of half open connections is to increase the value for 8 to 50 to 60, too high or too low will decrease the download speed drastically.