Focus the upload bandwidth to as few upload slots as possible!
(only one, if the top slot wants it all).
Transfers files to fewer people at a time, but faster to each. Faster transfers makes chunks complete sooner, making it possible for other clients to share the chunks sooner. This gives more sources in shorter time, sharing the upload demands on several computers sooner. At 76 Kbytes/s ZZUL opens ca 6-10 slots, when official eMule opens 24 slots. ZZUL only opens new slots if necessary to use the configured bandwidth. Upload slot focusing version 2 is available in this patch. Version 1 is used in eMule Plus (and others?). You can see which upload that has the highest priority by checking the number in the "Slot #" column. Slot #1 get all bandwidth it can handle. Slot #2 gets any leftovers after slot #1 has taken what it wants, etc.
When you download a file, it is good for you to give all chunks you already have of that file to other clients as soon as possible. As soon as you have spread your chunks, the other clients will actually help you to download the chunks you are missing. Then you can get those chunks from them (and fast, since you now have good credits with them). It will also be easier for you to get the chunks from the original source, now that is no longer busy uploading chunks that you already have, to other clients. So set your upload speed as high as possible!