In GeneralMost people agree that the optimal SM list has 3+ formations on table to defend the SM table half and act as reserves / objective grabbers / scouts-containers / supports etc (Note NOT all these roles simultaneously!). This is often where Warhounds, Land-Speeders, scouts and/or Tacs-Devs come in. A usefull consideration here is Devs with Dreadnoughts and Hunter upgrade as a Garrison, placed in an advanced position to provide the AA umbrella.
Using 'Corners' makes the enemy deployment a little more predictable, which in turn assist you in planning where to land your forces. As Neal says, Planetfalling forces need to have very clear roles, and need to be relatively close to each other to provide support. Depending on when you intend to planetfall, Terminators teleporting into the rear of the target area can assist greatly (then picked up by a THawk for a 2nd round assault). Alternatively, Termies flying into assault in a THawk is also very powerfull . . .
The main point is you must have a very clear roll for each of the formations, and alternative strategies to cater for those situations where the enemy has thown a curved ball and is not where you want him (which is usually the case as far as I am concerned

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A/C- TBolts work best in two roles:- to ground assault broken formations, and to intercept other aircraft. If you put yours on CAP, it can deter the enemy - so you trade off each other's activations - but by not activating, you keep your options open (and preserve your a/c).
- THawks are multi-purpose units, excellent at assaults and recovering formations, but also good at gound assaults to shoot-up enemy formations and prepare targets.
- Landing Craft are THawks 'on steroids'! A bit more expensive, but even more flexible in their usage as they can also land forces in a nearby-supporting location (like a Black Templar Crusader formation or even Whirlwinds) to enable the Marines to mount a major assault with mechanised support.
- The main point about the Hunter is that it will place a BM on every enemy air formation, which is a significant help to you if it assaulting you, and also helps to prevent the A/c from returning next turn. If it shoots something down all the better, but don't bank on it (the same is true for TBolts). If you adopt this mindset, you realise that the enemy Air attacks will get through, so you need to make better use of cover to minimse the impact. The other main roles of the Hunter is to boost formation size, boost formation firepower and provide cover to adjacent infantry units.
Bikes Vs AssaultI am coming around to the thought that Bikes work better than Assault Marines in this style of army, because they are more versatile after the initial assault. Best deployed in THawk as a mobile reserve (though they can planetfall in a THawk if needed).
TermiesThe Marines most cost effective formation! Don't go out without at least one formation. Used to spearhead your assault they are usefull both to teleport and air assault - what is not to like? Always take a Chaplain, but do not be afraid to use them to shoot as 8x AT/AP 5+ will hurt all but RA targets. And as for costs, at 400 pts, this is the equivalent of an upgraded Tac or Dev formation.
Theory of TwosBy and large I would agree with TRC that you get a kind of economy of scale in E:A where you use two formations in support of each other. Two IG infantry companies within 10cm of each other are much more effective than in separate locations, and the same is true of most SM formations. I quite like to use LC with Termies+Dreads as a major assault, supported by a 2nd termi formation in a THawk to capitalise on the assault. Taking this as the basic principle, you then time the planetfall around this, using Land-speeders, scouts and other on-table formations to pin / 'shape' the opponent.
But in the end, it is your plan and you should play to your style.