This question is an attempt to find an abstract answer to every "one word or two?" discussion.
My problem is exemplified by this scenario:
My text editor's spellchecker recently corrected me on my use of "video game" because it felt "videogame" was proper. Searching the Internet, however, led me to feel "video game" was proper, despite there being many credible sources using "videogame". As I'm writing this now, the spellchecker in my browser wants me to use "video game".
Along my search, I found a quote in a forum from a Wired magazine editor saying they always join words whenever possible. That person credited their book, "Wired Style: Principles of English Usage in the Digital Age", for that decision. I thought this was interesting.
It seems it's up to me to decide what form I want to use. How can I make such a decision? What factors should I consider?