This is sort of a weird question, because Soul Fire isn't really a bread and butter ability in the end-game warlock (it's cast time is simply too long to garner adequate DPS). There are a few situations when it becomes useful to cast however.
Destruction (Destro) Warlocks have a talent called, appropriately enough, "Improved Soul Fire" which provides a damage buff once soul fire is cast. This buff makes Soul Fire an acceptable spell to work into a destro rotation (once every 15 seconds, whenever possible)
They also have the "Emberstorm" talent, which simply reduces the cast time from 4 to 3 seconds.
Demonology (Demo) Warlocks have a talent called Decimation, which triggers when you hit a weak enemy with one of your primary nukes, reducing the cast time of soul fire by 40%!
Improved Soul Fire and Decimation are mutually exclusive, (they're too far up their respective trees for off-specs to reach... at least until the next expansion), but Emberstorm is not.
Because you want to maximize Soul Fire, I think you should go at it from the Demonology angle because, unlike Destro, once mobs dip below 25%, you can start chain-casting fast soul fires (destro's buff doesn't stack with itself, so there's no way to get use out of more than one soul fire / 15 seconds).
When questing, you may be able to simply soul fire mobs to death, the longer cast-time allowing your Fel Guard to build a better aggro buffer, and, if you're in a particularly thick area, you may be able to keep the Decimation buff up indefinitely, meaning you can just cast soul fires willy-nilly.
Specifically, for Questing, I would suggest this build (you said you were approaching 60, so add more to the tree as needed): http://www.wowhead.com/talent#IZcbrfMu0R
And for PvP, I would caution you that lower level PvP isn't representative of end-game PvP, and then suggest you use your dual-talent spec for something along these lines: http://www.wowhead.com/talent#IZIrrbozbR
Hope that helps!