Give them something to start with
I've found fantastic responses when students are given the first part of a sentence to complete - it's like it just unlocks the door to reflection for them...
I put the following slide up on the screen and ask the students to pick 1 or 2 sentences to copy & complete. This could be done mid lesson or at the end. Sometimes I discuss the responses verbally with the students as part of the lesson, otherwise I use them as something to respond to and establish a dialogue with while marking books. Given these starting points I've found that the quality of responses is usually really good.
(A quick health warning here - the content of the picture is NOT mine originally. I picked it up somewhere on the web, liked it, tried it and found it to be really useful. However I've forgotten where I got it from originally. Based on the powerpoint file I believe it was put together by someone called Julia Fardy - thanks Julia! Really not looking to steal someone else's work here but it's so useful I want to share it! - if anyone wants to claim credit for this I'll happily reference properly...)
Something else I've found useful is to ask students to "tweet me about their progress" - currently they don't do it actually on twitter, just write it in their books, but long term I'd like to think we can get to actual tweets.
The first time I ask a class to do this I usually have a prompt like this:
(This slide is mine but I'm sure others use progress tweets as a concept)
However they soon get used to it and now with my classes I can just say "do me a progress tweet" and they give me some useful feedback.
Interestingly the 140 character limit seems to spur them into actually writing more effectively than a basic "What went well". Again sometimes I will discuss them verbally, or use them as part of a dialogue in marking. It's also really nice when the students come up with a few #hastags that relate to the lesson or key words.
Both of these ideas can become instant reflective plenaries that can be bolted into a lesson with minimal planning. Particularly useful if a lesson has taken an unexpected turn off piste and your planned plenary wouldn't work any more.
Key point though - it's vital to show the students that you value their comments by discussing/responding to them - otherwise they'll stop putting any effort in and the comments stop being so valuable.
Give them a try - they're well worth a go.
All comments welcome as always.