While probably not as important nowadays than back in the (slow) cyberspace, I like to tweak my torrent client.
The most important factor is whether your client’s port is forwarded or not.
In order not to flood the upload speed, a good rule of thumb is to use 66% of maximum upload bandwidth 1 :
With a 100 Megabit/sec connection, one should use (100,000 / 8) * 0.66 = 8250 KiloBits/sec.
For other metrics like maximum number of connections per torrent or maximum number of connections globally, I use the Azureus U/L settings calculator. I enter the Upload speed of your connection and Number of torrents you usually download simultaneously (under Preferences).
For the seed, etiquette says 2 :
It is common courtesy to seed at least 1:1 so you are at least giving what you are taking.
Anything between 150% to 200% is probably fine.
For alternative speed settings, I use 60%-70% of the maximum upload/download bandwidth and take 3 digits out. With a 100Mb/s connection it limits torrents to around 1Mb/s.