I have the exact same problem whenever I sit down to write.
I like the idea of placeholders (like Lauren suggested) and I use that a lot. If I find myself stuck while writing a sentence, I just add some placeholders or, more often, asterisks that refer to notes on a separate page. If needed I add simple sketches/illustrations to my notes so that when I come back to elaborate on that part I can recover the scene more quickly in my head.
At first it is a little hard to get used to this process (if you are like me) because you will feel that by not righting the perfect paragraph right away you are leaving a hole behind (well, you are, in a sense). But once you get used, it is a real relief that you can go on unloading the ideas your head more fluidly.
Another technique I've been trying is to first write a scene in the form of a script (play or comics format). This way I can quickly describe to myself the whole scene, the characters and a rough approximation of the dialogue I want. This allows me to review the scene iteratively while adding annotations to the narrative elements separately (e.g. dialogue, scene description, point of view, etc). It also helps me to get a clear picture of the scene almost like I would see it in a movie.
CAMERA (or point of view): on the bridge, facing the opposite of traffic direction. To the right and above a man can be seen preparing
to jump from the top of the steel support. A few pedestrians notice
the man and stop to look. Some are taking pictures. One of them, a
young man, is protecting his eyes from the sun with his hands while he
looks up and screams something.
CHARACTERS (important roles in the scene): a man ready to jump into
the river, a young man who stops to try and stop the suicide.
Young man screams: "Hey Jeff! It is off! They are not after you
anymore! They are leaving you alone! Don't do that!"
Jeff (the man ready to jump) screams back (hard to hear due to the
strong wind): "You are lying! I can hear them coming, but they won't
follow me into the water. They can't!"
Well, not a very creative example, but you can probably get the idea.