Recently BoLS posted an article on how doing something unexpected can throw off a player who has a "tailored" army list, one designed to beat a specific foe. There aren't that many players who have the money to pull out a new army whenever they want to. The simple way to get around this is to not tailor your list to beat any army but every army.
That last statement may sound strange, but let me explain. Most armies have some things in common, for instance most transport vehicles are AV 11 or 12 and can move up to 12" per turn. Most armies have some sort of punishing CC unit or some monstrosity that can run rampage in infantry squads. On the flip side, some armies have the ability to send out waves of models and field so many wounds it's tough to kill them all. What's a player to do?
First look at what your army doesn't do well, how each your opponents can play that you simply cannot compete with. Look at how these armies fight in that manner. Then look at how you can best bring that threat down and include that in your army. Don't specialize your army, look at weapons and units that can perform multiple tasks in your army and include those units over units that can perform one task very well.
For instance, you're fighting Tyranid monstrous creatures, IG mechanized veterans, and a rhino rush Khorne army. What would you need to deal with theses armies, without changing your lists? One solution that springs to mind was posted by Stelek over at Yes The Truth Hurts in his "Fast and Slow" Marine build. this list focuses on bike squads with two meltaguns and a multi-melta attack bike supported by dreadnoughts with dual twin linked autocannons. The high strength of the autocannon makes it a very good weapon to kill transports and allows it to wound most monstrous creatures on threes and infantry on twos. Melta weapons do this even better, can harm AV 14 and penetrate any armor.
Just food for thought, but what do you think about bringing a specialized army as opposed to a generalized army?