Unfortunately I imagine that our raids strategy for this boss is likely atypical, as it involves with a single hunter being responsible for Magmaws adds. Additionally I'm going to assume that anyone taking the time to read this is at least somewhat informed concerning the fight mechanics, as such the advice I give from here on out will consist of small observations on my part and things that I have found particularly useful in this encounter.
As I said, our hunter is responsible for all of the lava parasites, as such, he is the only player NOT in direct melee contact with Magmaw. This solves a few problems and creates a few new ones. In terms of pros, now only one person is required to move when the lava pool spawns beneath them, however, it is now imperative that the other 9 players get the hell into melee range as even a few steps too far out can cause the parasite pool to spawn nearly directly upon the 9 man melee pile. If for some reason the Hunter has trouble kiting all of the adds, first we yell at him, failing that, our Fire Mage will supply an aoe slow or if for some reason HE is dead a warrior will go and piercing howl the adds.
My favorite time to use Power Word: Barrier during this fight is right after one of the head down phases, just before he breaks out of the chains. Reason being, people are easily confused! They need to stack back up in melee IMMEDIATELY not only to avoid putting the parasites somewhere the Hunter doesn't want them, but because when Magmaw breaks free he immediately Lava Spews, so you'll get a lot of mileage out of PW:B there anyway, so why not help direct your raids attention to the right stacking location at the same time?
Once you've hammered it into your raiders head that they need to maintain melee range to Magmaw, the fight becomes a breeze. There are only really 3 sources of damage. Melee damage to the tank, damage from his 2 target at a time lava bolts, and the raid wide AOE that all healers hate, Lava Spew. Personally I keep a shield on the hunter and tank as they are our MVP's for this boss and just smite/holyfire the ever loving hell out of Magmaw. Having the entire raid stacked helps with positioning with Power Word: Barrier and leaves us all standing in any aoe healing zones our comp may have that week. Help bring the raid back up post lava spew with Prayer of Healing and its back to Smiting/Holy Firing. Pain Suppression can see some use during this fight as well while Magmaw is devouring the tank. Popping your Shadowfiend during a head down phase is preferable, as you'll get the most damage out of him, doing so right before Heroism will sweeten the deal just that much more.
That's it for my Magmaw overview, certainly not a comprehensive guide but maybe someone will take something away from it.