◊ Shadowed

That macro abuses the fact that CancelPlayerBuff isn't secure, so all it does is.

1) If the cooldown is 0 (Not on GCD), we have the mana to shift, and it's usable then cancel the cat form buff
2) If we are still in a form, stop the macro
3) Cast cat form

In the macro that jon3k gave, adding "!" before the form name in /cast will make it so it only recasts the form when it can meaning you could spam it while in bear form for example to reduce how long you're in caster and making it harder to be caught in caster form. Something to remember, is that if you have auto unshifting disabled (CVar autoUnshift) then the macro jon3k gave won't work as it should, instead of reshifting you when it can it'll cast the form only if you aren't in it already, type /script SetCVar("autoUnshift", 1) to make it work like it should.
Depending on your preferences you can just make macros for each form (or modify your existing ones) to look something like the below.

#showtooltip
/cast !<form name>