Going for Religious Dominance in Civilisation 5: Gods and Kings

The key to religious dominance is spreading early before other religions start popping up. As Paralytic notes in the comments, spreading into an established religion requires tremendous effort that is often not worth it. Instead, you want to be the established religion that requires tremendous effort to dislodge.

So basically to become a dominant religion, get to a religion as fast as possible, and get missionaries out there ASAP. You want to primarily target central cities that have many others within passive pressure range. Get that pressure working early. Then you solidify that with one of the two Enhancer Beliefs that improve passive pressure

Itinerant preachers     Religion spreads to cities 30% further away 
Religious texts         Religion spreads 34% faster (68% with Printing Press)

You want an interlocking grid of religious pressure, so that even when religions start popping up, they have difficulty spreading against the wall of cities already converted. For example, see Shaka here:

I got to his cities (and the surrounding ones) early, before he founded his own religion. Even though he spent tons of effort converting his cities to Buddhism, they all converted back to my religion due to passive pressure from all the surrounding cities.

Now just to the southwest of the previous picture, I did not get here early enough:

While the eastern portion of Poland is my religion (and will stay that way due to interlocking pressure from across the channel), I was too late to reach the central area, and so I'd have to invest a lot to convert that. (Well...technically I do have 5000 faith to spam him with missionaries, but that requires a lot of effort at this point)

As for the reward for your early spreading effort, you'll probably be looking at one of the following Founder Beliefs (of which you can only get one, unless Byzantium)

Ceremonial burial   +1  Happiness for each City following this Religion 
Church property     +2  Gold for each City following this Religion 
Initiation rites    +100  Gold when each City first converts to this Religion 
Peace loving        +1  Happiness for every 5 followers of this religion in non-enemy foreign cities 
Pilgrimage          +2  Faith for each foreign City following this religion 
Tithe               +1  Gold for every 4 followers of this religion 
World church        +1  Culture for every 5 followers of this religion in other civilizations

Which one to pick will be game dependent. In the above game, I'm running Tithes, and am making ~200 gold from it. However...in the words of someone who's better than me at this:

Ceremonial Burial just always seems the strongest of the beliefs. Think of it in hammer terms. CB per spread is the equivalent of 50 hammers worth of colosseums. Compare that to Tithe or Church Property which are worth more like 25 hammers (a quarter of a market), World Church at 33 (a third of an amphitheater), Initiation Rites which is 21 hammers (one-fifth of a bought settler.)

-quoted from T-hawk