In my opinion, some of the "secrets" to making decent movies are :-
Know your limits; try and push the envelope a little with each project, but don't try and reinvent the wheel if you don't need to.
By the same token, don't expect to make something as visually stunning as say Coming Home, or Expedition, or Strife if you only picked up the software a week ago and have 5 mins to study it before trying to make your movie; you'll end up sadly disappointed ;)
Do what you're good at, get help for the bits you aren't. If you can do great voices but can't mod to save your life, ask for help. If you can do great camera work but can't write, get a writer; vice versa if you're good behind the keyboard but can't line up a shot to save your life.
If you can't get what you need to make a movie work, scale it back or change it (unless that totally compromises your "artistic integrity", of course! :P)
Be realistic; if you want it to look and sound right, you're either going to have to a) spend a loooong time on it or b) be an audio/technical wizard or c) both! You can't make a silk purse out of a sow's ear, my dear old mum used to say, but you surely can do the other way round with very little trouble!
If you get help for anything, be specific about what you need and when by. If you want VO help, be specific about what format you want and when you want the files back by.
Be ready to use and (if necessary, break) all the "standard" conventions of movie making; what works in a film studies book about "optimum positioning" and "golden ratios", etc, doesn't always work for machinima. Be prepared to get creative to fix or avoid problems with sets, animations, lighting, etc.
And, above ALL else... have fun and enjoy the process, or it's pointless you indulging in it! That's the biggest secret, after all ;)
