installing multiple distributions on same system and recycling

Hello! I just bought a new hard drive and I am going to install 9.1 on it. I wonder what I should pay attention to if I want to leave the possiblility to test other distributions later on and to be able to reuse some stuff: at least files but maybe some installed applications... My questions are therefore: - What should I pay attention to to be able to install and use another system alongside (parititions, ...)? - What can I (not) reuse from one system to another (files, applications, configs, ...)? Thank you for your help! Patrick