Remember to share files of value to other people. This is a p2p network and it only works if people share. To only take and not return is not only bad etiquette, its bad for the network.
It is also very detrimental to the network when people use leech monitors such as LeecHammer or MxMonitor to set unreasonable share requirements for those that download from them. This can discourage new users, who may not have as much to contribute, but are willing to make a vital effort.
When we act in such a miserly fashion, it can only lead to stagnation, which will inevitably lead to the end.