Keeping your computer running nicely is simple.
Just consider a computer as your solid waste explusion port.
Would you:
Stick anything that someone mails you in there?
Stick something that runs in there?
Hang it out a door on a busy city street?
Leave it unattended in a public place?
Trust your friends to use it?
Use all kinds of mechinisms and chemicals to keep it clean?
Store things you really don't need in there?
Would you like it searched before being allowed to use something?
Would you put allow someone to put surveillance equipment in there?
Put something that's too big for it in there?
And remember, just because it makes you happy and feel good doesn't mean it belongs in there.
There, simple.