Here was the source image everyone was given to work with:
The first thing I noticed with the image is that it's a lower resolution tumblr pic. That doesn't leave much real estate for adding text without modifying the image to add some. I chose to remove the blur of red in the top right corner and stretch the shadow on the right to make more space.
The image itself gave me the feel of Snow White and the witch's hand for some reason, so I ended up on a bit of a fariytale slant. Somehow, I settled on a reverse Pinnochio theme...real boy becomes puppet girl. I added a frame to the image to give the feel of the classic storybook illustration brought into real life.
That left the writing. It's the most important part of the whole caption to me. The words should mesh with the image just enough to tie them together while letting the image speak for itself. The controlling shadow hand in this image is ideal for themes of control and loss of it.
Since D/s themes can sometimes become heavy, so I chose a lively and fun font to keep things light. It may not be the most readable font, but it expresses the undertones of softness and femininty I want the hidden dominant to have.
I like to add a teaser title to captions when I can to give a quick taste of the full caption. I will usually put that in some 'dead space' in the image where it won't interfere with the core focus of the caption.
Last, and hopefully least, I add a transparent watermark to the caption to help deter other people on the Internet from claiming credit for my words and artistic adds as their own. If this hadn't been a problem in the past (see here), I wouldn't bother. Sadly, there are schmucks out there who scrape and repost captions while claiming they did the work.
And here is the final result:
Have questions on something I didn't cover in my explaination? Ask them in the comments, and I'll try my best to answer.