So i've been watching some fancy youtube videos of people making things with redstone. One thing that catches my eye is people are able to shift-click to place blocks/etc on blocks with an inventory while flying and not descend! It annoys me that I descend when trying to put a lever on a dispenser or something similar while flying around..
I realize this post is from 2014, however I (in 2020, year of the great coronavirus) found this unanswered question in the first result of my google search for the same question. After a little more googling and eventual redditing I eventually found the answer: hold space (jump/ascend) at the same time. Holding the ascend key while trying to descend will leave you just hovering there. I find that I end up slightly going up or down though depending on which key I press first. Hope this helps someone else in the future who was stuck like me.