I've been traveling around Iowa a bit lately to lead professional development workshops on standards-based grading.  Usually after an hour or two, a teacher in the group says something like this:

I agree with everything you've said so far.  Although it sounds like a lot of work, I think it makes sense.  How do I communicate this with parents?
My response:
Communicate this idea with the students first.  If you don't, you'll be doing this to your students rather than with them.
I suggest teachers get students excited about the failures of the current grading and assessment system using the same illustrations I use to motivate the change with them during the workshop.  Here are a few examples.

------------

Bobby aces the homework, aces the quiz and aces the test.  He's the high flyer in your classroom, right?
Suzy doesn't really "get it" on the homework (75%) and struggles a bit on the quiz, too (80%).  Somewhere after the quiz, Suzy has a light bulb moment and "gets it" so she aces the test.  

Which student knows more? They both have the same level of understanding. 
Which student gets a better grade?  Bobby.  Why?  

Enter Jo-Jo.  
Jo-Jo bombs the homework (50%) performs poorly on the quiz (60%) and bombs the test, too (60%).  A week after the test,  Jo-Jo has a light bulb moment and walks into the room yelling, "I get it!" and can demonstrate a high level of understanding.   The old systems says,
 "Too bad, Jo-Jo....you didn't get it by my arbitrarily set deadline."  
The new system should say,
"Awesome, Jo-Jo..let me go change that in the grade book.  I'm so thrilled you learned it a week after the 'test.'"
-----
After getting students fired up about changing grading and assessment, it's time to pitch the idea of feedback focused on specific learning targets.  A volleyball/coaching analogy works well.

When you're a bench warmer and you want to become a starter, you say, "Hey coach, how do I get better?"  You do not expect him/her to say, "You stink" or "Get better."  A good coach says, "You need to get better at spiking and digging even though you've got a pretty good serve."  
Why do our tests, quizzes and projects only have a single letter grade or percentage attached to them?  What if feedback could be even more specific...like the good volleyball coach?

Letter grades just say "You stink" or "You're good."  We can do better.

(This story can be modified to use other sports or vocal/instrumental music)
---------------------
These types of 5-10 minute conversations over a few week time period can really get students excited about changing the culture and grading/reporting system in a classroom.  In my own experience, students eventually were begging me to put these wild and crazy ideas into practice.  When that happens, you've started shifting the ownership of your classroom over to the students.

...and that's when the fun really begins.  

Feel free to share your successful standards-based grading student conversations in the comments below.

(Oh, and if you're still interested in communicating with parents after the fact, here's a place to start.)