I try to avoid it. My family used to say Gesundheit when I was growing up, so that's my default reaction. But I've had some people not know what on Earth I was saying, I guess it's a regional / family thing.
If I'm speaking to someone who knows another language, I'll usually say it in that language if I know it (Salud — for "health" and not "salvation"! — in Spanish, "À tes/vos souhaits" in French), or see if I can get away with not saying anything. If I can't avoid it, I'll say Gesundheit or rarely Bless you, but never "God bless you".
My Social Studies teacher says "Be healthy". Could always go with something like that: sideless. Could also not say anything, but I've known people who stare at you if you don't say anything and keep staring, evily might I add, until you do say something.
To answer the original post: I say gosh. Kindergarten, yes, but it works. Or curse. Cursing always works :)
It's true that some people expect you to say something. To be honest, I kind of do sometimes, too. If I sneeze and no one says anything, occasionally I will momentarily feel offended and then tell myself that there is absolutely no reason someone should say Gesundheit or anything else (and certainly not "bless you"!) just because I sneezed. "Be healthy" is interesting; I wonder where your teacher got that one.
I am an accomplished blasphemer, as well as frequent user of profane language. I don't think it's a horrible habit, just a habit. I do make an attempt to hold back when I'm around children, people in positions of power over me, or those whose feathers would be ruffled by wee swearies. Doesn't always work, though. I don't always have conscious control over it.
"Oh. My. Gawd!" for news that leaves me gobsmacked.
"Shit!" when I've accidentally dropped something on the floor.
"Goddamnsonofabitch!" when I've stubbed a toe, or some mishap with a similar level of pain, but no permanent damage.
"Goddamnsonofafuckingbitch!" when I've physically hurt myself and at least achieved brusing, a cut, or a scrape.
The occasional "Jesus, Mary, and Joseph!"
"Je-zus-fuck-ing-keerist!" when generally frustrated, or when having to deal with bureaucracy.
Some of the above are interchangable, depending on what the Rolodex in my mind serves up.
When something really gets me, catches my by surprise 'big time', and makes me angry, I'm not artful or articulate. I just say "Oh fuck".
In settings where I can't cuss, 99.99% of the time I have better control of myself. Sometimes I will say "I can't tell you the word that I'm thinking, but you can guess what it is".
A few weeks ago I was involved in a highly physical situation involving someone who had a psychotic break. You would not beleive how strong such people can be, and how quickly they can injure themselves and others. In this case, the individual had made multiple actual strangulation attempts. Working in a settting that is very majority women, sometimes I swear that this is one of the reasons that they keep me around. I had to wrestle this person to the floor, and hold down until we got a sedative on board and get the medics there. In this situation, the word "fuck" came out involuntarily, multiple times. Just no way to think of something clever. I had to make some apologies afterwards.
I really admire people who can cuss colorfully. When I hear an expression like, "He could fuck up a one-car funeral" or "She's as useless as hen shit on a pump handle", I want to give the speaker a big hug. I even recall these expressions, from my youth. I just cant think of them in real-time. If I could, I'd be as happy as a dead pig in the sunshine.