After some thought was put into it, I decided that the Boa Constrictor would be the ideal choice. Though of course, I was mindful to think the wish through first, I did, after all, want to retain control. Thinking it through, and settling on the word choice, I made the wish.
"I wish I could turn into a Boa Constrictor, but I can control it at will, able to turn either partially or fully into one and back. I will also retain the abilities of speech and hearing, and be able to see in the visible spectra." A soft buzzing sensation ran through me for a few seconds then faded.
Now I decided to put it to the test. I mentally commanded my skin to change into scales, which they promptly did, shifting from pale skin to rich earth green and brown diamond scales. And they felt wonderful. Next my hair faded away as my face pushed out slightly to form a partial snake head, though I still had a nose, it was splayed out a bit, and my eyes now carried vertical slits. The ears were gone, replaced by small holes, though I made sure my hearing didn't diminish. The nails on my hands lengthened a bit and sharpened, and were now black, as were my toenails.
Satisfied for now, I took a look at myself, sleek, predatory, my stretched mouth filled with sharp teeth that I had allowed to remain. The sensation resulted in a stirring in my loins as my new, scaled shaft filled with blood and rose.
'Ah, missed a step, but no matter, I can always correct it later.' I though to myself as I gently rubbed it and my balls, the new surface sending waves of pleasure pulsating through me, as I hissed in pleasure. Soon, the inevitable occurred as hot, serpentine seed blasted out and onto my chest.
A couple of minutes later, I recovered from the high and pulled my wits about me. I cleaned up with a good shower and dried myself off thoroughly. I decided to put on a silk dressing gown for now, and set myself down on the armchair for some TV.