Of course, before you even begin thinking about how to best organize a strike against Earth, you need to ask yourself whether it's worth going to that kind of trouble.
We don't always know why the aliens want to kill us. In some cases, the question remains open. Sometimes they just enjoy it, like in The Day the Earth Stood Still, where the aliens are left-wing extremists out to stop global warming. In Skyline, they needed our brains for some reason. But in most cases, they just seem to want our resources. In Independence Day and the upcoming Battle: Los Angeles, the stated goal of alien invasion is resource acquisition -- they want our shit.
Thing is, if you have the technology for space travel, space mining and space ExxonMobil, there's really no reason to attack Earth for the same resources that you'll find on Mars. In fact, this solar system alone has eigh- seven planets (sorry, Pluto) that won't shoot at you, and they're all made of the same stuff. In fact, you can even mine the moon, and there isn't a whole lot that the landlocked denizens of Earth can do to stop you.
Come on, ISS. Bring it.
But just say that nothing short of Earth's unique ecological profile will meet your needs. Maybe you want water (unless you're from Signs), and we admit that's pretty rare in the universe. But maybe you should take a look at one of the universe's many "Goldilocks planets" (so named because they're not too hot and not too cold). Statistically speaking, this galaxy alone should be littered with planets just the right size and distance from their suns to develop liquid water, an Earth-like atmosphere and all the raw minerals you need. And either all of them or most of them are devoid of the kind of life that can shoot missiles at you and hack into your mother ship.
These movies are usually trying to parallel the reasons we fight wars here on Earth, and while we do fight for resources, we have the good sense to secure uncontested minerals before we consider waging an expensive and violent war. You can bet mankind will be mining Mars before we consider sending warships to Zeta Reticuli.
THERE ARE BETTER WAYS TO MINE.
Really, guys, this stuff is just common sense.
You aliens should really pick up our new book. It'll serve you right when you're attempting to demolish humanity.
For more messages to our would-be rulers, check out 9 Simple Requests For Our Robot Overlords. Or find out why Skynet will never win, in 4 Reasons Terminators Suck At Their Jobs.