When you leave a malaria-affected country, donate your mosquito net to a local church or other helping organization or other religious institution, they can distribute it to a family who could use it. A simple action like that could save a life. Malaria is still killing people and poverty is partly to blame. The use of these bednets has helped reduce the number of people getting malaria (and other serious diseases).
Make sure the representative of the organization you give it to is aware of the issues like making sure there are no holes in the net, the need to tuck it in properly when it is being used, to not let it rest on the skin (because mosquitoes can bite through), how to hang it properly, how long the insecticide of your bednet lasts (the insecticide makes them more effective) (bednets can be retreated), how often it can be washed without removing the insecticide, ensuring none of the parts present a choking hazard to children. This is not an exhaustive list of issues.