Well, I tend to tread VERY lightly when using humiliation in a caption for someone else, especially someone that I do not know well. Usually I take into account how they've responded to (1) other captions made for them by others, and (2) if/how they respond to captions I've made for others. This is where people leaving comments pays huge dividends. If a person has responded well to something I've done, then I am more likely to target them in a future caption AND I have a decent guide to what they are looking for.
Preferences are even more important when working with humiliation. Not so much in what people like, but in what they DON'T like. I would rather make a relatively safe caption, then one that goes too far. I can always push the envelope in future captions for them. For instance, with Jennifer up above, I know her really well, and I'm pretty sure that I know exactly what will make her shiver and where the line is that I need not cross. I got to that point by becoming friendly, chatting in YIM, and just comments we've left for each other through the years. There should always been an open line of communication involved.
How to put across the situation in a smattering of words? I would suggest using a picture that explicitly gives a huge amount of background so that you can save on word space. Bondage pictures would imply being forced to do something, though even just having the girl on her knees or below another would show submission of some sort. A public photo would already lend itself to embarrassment scenarios. For text, it would often be better to just have dialog between the humiliated and the dominant, or the humiliated and someone that person knows. There is no real WRONG way of doing it though.
I do have more to discuss, but I am curious to see what others have to say before I elaborate more, as someone might do so more eloquently that I could.
Lastly, I thank you for thinking this blog is an interesting idea. As I've said in the past, I love creating captions and I don't think the craft gets enough attention. Its a vehicle for fantasy, but often the best captions are those that make you think, whether it is conscious or not. I'd like to think my captions appeal to more than just a prurient interest, and that the composition is done correctly, instead of just slapping some words on top of a smutty picture.