Towards the end, prior to my recent retirement, I always did a "before" drawing or concept. Then when they were finished with the assignment they had a lot to talk about since they could see their progress. Their reflections were more meaningful as they could ponder the technique and process as well as the outcome. So for example if I planned on doing instruction on face proportion, I would have them spend one period drawing the best face they could (as their before), and then at the end of the process of me instructing them, and their exercises and then the final work, I would drag out their "befores" and boy was there a noticeable difference. (As an aside I started the "before" and "after" portfolio because if merit pay comes to pass, I don't think a final "after" portfolio will do art teachers any good, since they will most likely be evaluated by someone who hasn't worked in the field, or if they have might evaluate someone's work based on a standard that does not reflect the student's ability. If you have a "before" and "after" then one could evaluate if learning took place. IMHO)
Another trick I would do especially with beginning students is at the end of the project, I would hang up another classes' work and put gigantic numbers over the signatures. The kids would then critique another class' work but would know what to look for since they also had the same experience. When you talk about others' work you are essentially reflecting on your own.
And lastly, students are reluctant to "reflect" if they do not see the connection to the next work you will be doing with them in class. If we are just a "make and take" profession, the "reflection" is of no deep meaning, again IMHO.