I'm curious as to the opinions of you top notch players out there on the use of religion in general. Specifically, how do you make all of your cities the same religion? (forgive me if that's a stupid question) Also, does "increased" religion influence culture much?

What's the best way to keep culture level high? Thanks!

I prefer to found a religion. Normally through building the Oracle - Confucianism or Christianity.
Whatever you found, make it your state religion, the religion should spread to your other cities, if it doesn't either build monasteries and then missionaries. Or change to the civic that allows building of missionaries.
Also AI civ units can "spread" your religion to their cities.
Your state religion gives 1 culture in each city it is present.
You can do the above if you adopt another civs religion as well.
However I like to have my own religion as the religion has a holy city, where a great prophet can build a holy shrine. This shrine allows the owner to see all the cities that has the religion and also receives 1 gold per city with the religion. The shrine also provides culture and spreads your religion more.

Other religions can spread to your cities but I find these to be a bonus, as you can build more monasteries and temples. It is best though to have as many of the 7 "holy cities" as possible with shrines, as you can generate lots of gold.

As to culture, other that the 1 culture in each city that has your state religion, temples, monasteries and cathedrals all generate culture. Each religion has one of these buildings and the religion is required in the city to allow the building to be built.

There are many buildings and wonders which provide more culture when built in your cities, especially the theatre. The theatre allows the placing of pops as specialist artists. each one of these provides a culture boost. Ultimately a great artist may be born. The great artist can be added to a city to provide a big culture boost or can be utilised as a culture "bomb" to provide a single one off boost.

Also the sliders in the top left hand corner of the screen can be altered to provide multipliers to culture in increments of 10%. This can be used once Drama is discovered. This is good if going for a culture win, however is not so good normally as it reduces science and gold.

There is a leader trait which gives +2 culture in each city and cheap theatres, however I think this is very strong at the start of the game and extremely weak at the end of the game.

I tend to want to increase culture in new cities (either built or conquered), which I do by sending missionaries of my state culture and by building theatres. The culture "bomb" is brilliant for large newly conquered cities.
I also want good culture in my border cities, Monasteries and theatres are good for these.