Writing about complex ideas without getting stuck I'm an experienced researcher and find it relatively straightforward to write technical papers. However, I also sometimes want to write papers that present a complex idea, such as a novel synthesis of existing work, or a question that hasn't been clearly formulated in my field. In writing papers of this kind I often start to feel "boxed in" by my own writing and unable to express the idea I really want to convey. It seems like this should be a relatively common experience, and I am wondering if there are known techniques to avoid or overcome it.

What tends to happen is this: I start out with a 'big idea' that I want to write about, and start working on an outline, but I quickly get bogged down. The questions behind the idea need to be motivated, and the idea needs to be placed in the context of existing work. In writing about these things I find myself getting drawn along tangents, writing about side points that I didn't want to write about in depth (but which now seem necessary in order to motivate the work) or feeling like I need to review whole other fields of study that I'm not an expert on. Often I end up giving up on the draft, because it doesn't seem possible to clearly make the point I was aiming for.

There are essay papers I admire that don't seem to fall into this trap. They are written in a way that always seems to push toward their central thesis, even if they have to make a lot of side-points in order to get there. I even manage to pull this off myself sometimes - I just don't seem able to do it repeatably.

I'm confident that the ideas themselves are well enough developed to be worth publishing. I've developed them over years and can easily express them verbally or in slide presentations. But writing papers driven by complex ideas is a difficult craft in which I have no formal training. 

I feel that the issues I'm hitting must be known and avoidable: What techniques are available to address them?
I am not sure what you have tried, so bear with me if some of these techniques are obvious to you:


Write from an outline: Complex matters need to be broken down. Writing from an outline helps with focusing on one thing at a time. It also helps with making sense of the big picture without having to go down each and every rabbit hole simultaneously. Remember that your outline isn't set in stone; on this consider the points below.
Explain your ideas verbally: You seem to be struggling with putting words on paper, perhaps because putting words on paper means fixing them. Explaining something out loud takes this pressure away, as everything can be qualified and corrected the next second -- you develop a sense of flow and can focus on the idea rather than the phrasing. Either talk to colleagues, or just to a mirror, a pet, or your left thumb. Programmers call this rubber duck debugging. You may find it useful (or maybe too restraining) to record your voice. I like to move around as I explain stuff to myself, and I had some good insights while running (alone and with a friend).
Either write in draft mode or in edit mode, never both at the same time: Draft mode is all about collecting ideas. Get the word count up as quickly as possible. In draft mode, you want to collect the raw material out of which you build your masterpiece later on. Silence the critic that lives in your left brain, and don't allow yourself to polish your draft yet. If you're really stuck, you may even try some free-writing exercises, like disabling the backspace key or writing with the screen switched off. Going off on a tangent is not only allowed but encouraged a this point! When later on you switch to edit mode, the first thing is to identify the key ideas that are now buried like raw diamonds in your draft. Highlight them, relate them to your outline (or write a new ex-post outline from them), and then structure the rest of the text around them, as you revise. Now, decide which parts are tangential and which are central to your argument. Cut the former and develop the latter. 
Iterate! You will probably never completely avoid tangential thoughts proliferating in your draft, but you also can't tell which is which before you have written them down in draft mode and critically evaluated them in edit mode. As you progress through iterations of drafting/editing, the tangents become less. Before you ask: Yes, this takes time. Thinking does. None of the papers you admire has been written in a day, in one go.
Never delete big chunks of your draft once and for all. Instead, save the draft under a new file-name or use a version control system and make regular commits. This allows you to go back to earlier versions if you lost one of those raw diamonds somewhere along the way. It also permits you to be bold with revisions and deletions, making your text more concise and clear.
Reconsider the boundaries of your project: As you revise your draft, you may realize that what you originally considered a monolithic project is actually stuff for two or more projects. This is a good time to go back to your outline and split your draft in two: Decide what you want to focus on now, what you might look into next, and how these parts are related. The latter is part of your concluding section ("further research is needed..."). (If you will, this is point 4. on a meta-level.)