Tell your students not to worry about completing their introduction first. Many students struggle with writing introductions, and they often spend a lot of time trying to correctly word their introduction that could be better spent working on the main body of the essay. Tell them that if they can complete their introduction first, then that is fine, but point out that they may find it much easier to do after completing the body of the essay.
Give them some good strategies for producing a good introduction. Explain to them that the introduction needs to grab the reader's attention and bring them into the subject. This could be done by explaining basic concepts or ideas that are central to the understanding of the essay, quoting an expert on the subject, or relating a real-world event that has drawn attention to the topic area.
Tell them to introduce their line of argument in the introduction. Introducing the line of argument at the beginning of an essay is like stating your hypothesis at the beginning of a piece of scientific research -- it explains how exactly the essay intends to look at the subject matter, giving a clear point to be proven or refuted through the course of the essay. Perhaps the essay is intended to dispel a commonly held belief, in which case the student can present the belief and explain the evidence he is going to look at to argue that it isn't the case.
Teach your students to conclude their arguments properly. Tell them not to simply re-state their idea as it was done in the introduction, and especially not using the same phrases. The conclusion of the essay should summarize the main points that were made in the body of the paper, and how effectively they prove or disprove the stated thesis.
Tell your students to discuss the wider implications of the points they have raised within the essay. If the essay is discussing a particular problem within society, they could suggest some potential solutions in light of the points they have raised, or simply state what could happen if the issue is left unchecked. Remind them to maintain a cautious tone when discussing what their essay has shown--they will not have proved something beyond all doubt, just added new information and argued a point of view. The conclusion should also provide a feeling of closure, tying everything in the body of the essay together so that it is complete.