How to clean install and only re-install old applications/files as you need them? I have Gooogled this for a couple of months now casually but have never gotten a guide that seems suited to me.

I am running Snow Leopard. It's really starting to get slow. Basically, most guides on the net either tell you how to do a clean install and keep all of your files, or none of them. 

What I am looking for is something in between.

I want to be able to have a completely fresh install, but then grab back stuff from my old backup as I need it. MS Office, maybe the occasional word file after that. Eclipse IDE, with all my original preference settings. Only as I need it. That way I can make more of less.

Does anyone know a guide online that resolves my problem? Or does anyone know how to do this themselves?

At this time, I am not interested in upgrading to Mountain Lion
I'm not sure that you have an issue. You are free to install formally or drag and drop any app from source to hard disk - if it doesn't meet that system's requireents, it will be sort of whited out and won't run.

Got good preferences files? Do the same thing.

Hope this helps.