Energy consumption

Refrain from buying new computers (a lot of energy and materials consumed at manufacturing time). Buying one also means discarding another.
Buy low consumption systems, in particular with “mobile” processors, supporting frequency scaling. Laptops consume 50 to 80% less power. On the other hand, their batteries contain toxic materials and they are more difficult to repair or upgrade.
Buy LCD instead of CRT monitors (consume 60% less and emit less mercury).
Switch off your DSL modem / wireless router / ISP box / printers at night or during vacations if you don't use them. These devices consume quite a lot (typically 15W).

Refrain from buying new computers (a lot of energy and materials consumed at manufacturing time). Buying one also means discarding another.