Struggling to write

One of my colleagues is struggling to write.  I struggle with writing too.  In the past it was much worse: I've come close to failing courses because of simply not being able to write.  It's really frustrating.  Writing is still painful for me and I'm by no means a writing guru, but for what it's worth, here is what works for me:

• Short blocks of writing time.  Sometimes I can write for several hours, but often I lose my focus after about 20-30 minutes.  I need to actually leave my computer to refresh myself.  Often I take a walk somewhere.
• Sentence at a time (even word at a time) writing.  Sometimes I notice myself stuck at certain point.  My thoughts are murky, I'm re-writing a sentence over and over again, or I'm not even writing anything I'm just feeling terrible about what I will have to write next.  It's at these times I forget about the rest of the piece and simply write the next sentence. As if after writing it I will be finished.  Sometimes I need to focus on writing the next word.
• Muck-writing.  I've just always called it this.  When I'm stuck I'll often open a new document and write something to throw away.  I make sure to hold myself to no standards of writing what so ever, just as long as I'm writing something.  Often I imagine it as my journal, so I speak to myself, and write questions to myself about the topic I'm writing about, and then I answer them.  After doing this for a bit I've often worked through some murky thoughts and I have some momentum for writing.
• Sentence push-through.  This is like muck-writing but on the sentence scale.  If I'm stuck at a sentence it can help me to just decide to write something, even if it comes out as something to throw away.  As long as the thought is there.
• Ad-hoc notes.  As I write I usually switch between writing prose and jabbing out point form notes about what I'm going to write about next.  The notes are just reminders to myself.  If don't write these notes down, I often get cluttered and stall, worrying that I'll forget the awesomeness I want to say next.  
• Having Conversations first.  Like muck-writing, sometimes I need to write a few emails or have conversations with people about the subject before I articulate anything in writing.  Sometimes I cover more ground when I talk.  
• Staying relaxed.  This is probably the most important part for me.  I often get really tense when I write.  I hold my tension by holding my breath or straining my own breathing, tightening my stomach, pressing my tongue up against the roof of my mouth, furrowing my brows, and raising my shoulders.  
  
  If I relax all of these points of tension I usually feel much more able and willing to write.  For me, relaxing takes real concentration and time: I'll often slip back into one of these "poses" without realising it.  I typically have to stop for a few minutes to relax every point and then I have to continually watch myself as I start writing again.

So after writing this, I'm curious to here from other folks.  What works for you?  I'm sure this discussion has been had many times over, I'd be grateful for links to writing resources you've found helpful.