In real, everyday life...I swear. I swear a lot.
On my OKCupid (don't judge) profile I say that I don't smoke, but "fuck is my nicotine". I can clearly the remember the day, a few weeks after my grandmother died when I was nine or so, and I heard my mother say "shit" for the first time. I remember gaping at her with that great "OHNOYOUDIDNT" face little kids get when their hero has just done something heinous. Until that point my biggest swear word was "sucks".
Suffice to say, I grew into more and more swears. My parents often comment on it (funny, considering they're the ones I learned it from eh.) It also doesn't help that many of my public domains, such as school and work, never discouraged it and most of my scholarly/working peers swear/swore a lot themselves. Swearing's even more prevalent on TV these days than it was ten years ago when I was still an impressionable little shit.
So, as you can imagine, my characters swear a lot themselves. But miraculously enough my narration always remains devoid of words that end with itch, it, and uck. In fact, my narrating voice is often seen putting on a "oh these characters and their swearing~" hat. If there is swearing in the narration, it's encompassed in quotes to imply that the narrator would NEVER say that, but the character sure did think it.
It's just one of those things I've noticed about my writing.
My narrator (who I often think of as myself, and I'm not sure how popular that is amongst authors...insights?) is the divine neutral. She (or he, or it, but since I think of the narrator as myself and I identify with feminine pronouns, there you go) sits back and omnisciently tells everything as neutrally as possibly, including the use of the most neutral language possible. But when it comes to matters of evil, she always has the subtle stance that she realizes these acts and thoughts are wrong and does not condone them, even though she attempts to communicate them to the reader as neutrally as possible. This is offset, of course, by my characters who are definitely NOT neutral and don't give a ~flying fuck~ about cursing everywhere if it's in their nature to do so.
In various discussions about writing I've come across throughout my years on the internet, the topic of openly swearing in novels is brought up a lot. Some people are very much against it unless the situation "direly calls for it, such as somebody just died", or feel that swearing characters depletes the approachability of their novel. Others don't give a flying fuck and happily let their characters ride the Swear Train to Swearsville, for it "realistically depicts the character's behavior". As you can probably tell, I fall into the latter camp. Most of my characters wear by default, unless it's specifically NOT in their nature to do so...and thus they spend a lot of time being abashed over the other characters swearing so much. The only work I have those types of characters in, however, are in Nagnomei, where many characters are raised in nobility and find swearing to be crass and beneath them. That's as intrinsically a part of their character as swearing is for characters who do so.
Swearing a lot does bring its own difficulties, however, especially in high fantasy settings where a part of world building is to develop a land's own language. Some swears translate naturally into Nagnomei, such as "ass", "bitch", "shit", and even "fuck" depending on the context, and if you approach some of the swears as a translation to what the character is actually saying. But, of course, there are some forced changes. Instead of saying "bullshit" my characters in that series often say "horseshit". And if there's any series where I just "fade to black" on cursing and cop out with "and then he said a string of obscenities", it's that series. Because sometimes I just don't know what they would say in that land that wouldn't sound out of place!
Hence why if I run a search of swears in any Nagnomei file, I do not find many. In CROSS//Rebirth alone, however, there are 84 "fucks", 62 "shits", etc etc. It's a lot easier to let the modern American swears ring out when your characters live in, well, modern America...and swear just as much as you do. Sometimes, even, it's just fun to sit there and childishly let your characters say all the swears you never got to say in front of your parents growing up.
This tangentially leads into another, slightly more serious field than just swearing - slurs, which a lot of swears fall under. Some swears are wonderfully neutral as to who they go against - words like "ass", "shit", "fuck", etc. Then there are the slurs, such as "bitch", "cunt", "retarded", etc. Swears that come from the oppression of minority groups, pretty much. I've cut many of these words out of my general vocabulary over the past couple of years, and I've deleted quite a bit of them as well from my writing - unless it really is time to drive a point home about a character. Although I've stopped saying many slurs in my day to day life, my characters still spit them out on occasion. Sometimes it's just a fantastic way to tell a reader "hey, this guy's a scumraking douchenozzle. look at what he's saying about women, eh." Sometimes characters are purposefully trying to hurt one another with words, including slurs. Some are probably even the type of people to go on their Livejournal and mumble about "political correctness". And some just don't care, especially in a time period (in this case, 2007 and slightly before) when using more recent slurs was still more socially acceptable than it is now in 2011. There are times when I am editing a scene in a novel and cringe at some of the dialogue a character is spewing and seriously consider changing it because it even makes me uncomfortable now - about half of the time I do change it to something more shitastically neutral, and the other half I leave it in based on the context of the scene and I decide that I want it to reflect (usually poorly) on the character. Even my heroes are not exempt from the "ugh, did he really just call a woman a 'bitch'" anger. There is also the balance of my own - and my characters' individual - privileges, but that is almost certainly a topic for another entry one day.
For now, let's just leave it at this: my characters swear twice as much during the month of November. I'm sure most of you can guess why.