1. Introducing the structure of the test, how it works, the timing of things like the different parts of the speaking test and some strategies to deal with the speaking test, etc. There are some basic errors that students can make that can cost them one or two band levels. Conversely, if the students understand these things and use proper strategies it can raise their score one or two band levels. (Photo: My students doing a mock IELTS test.)
2. Raising their English level. This does not mean teaching them a lot of complex vocabulary or grammar rules. One of the best things is to provide them a lot of English samples at their target level. If their target is Band 6.5 then provide a lot of samples of English at those levels. This is especially true of your own speaking to the students. Although speaking at Band 6.5 is characterized by a small amount of advanced vocabulary, making a lot of errors with basic vocabulary and grammar is a sure way for them to crash and burn in the test.
There is some great free material at ESLPod  but I would suggest you keep something in mind if you use it. ESLPod features some bits of more advanced vocabulary. This specific vocabulary may be more useful and attainable to your students in a passive sense. That is, they may only learn it well enough to be able to recognize it when they hear it or read it but may not be able to produce it when speaking. However, when evaluating ESLPod, do not overlook all of the other speaking and text in the podcast. It consists of a lot of basic vocabulary and samples of grammar. Think of the podcasts as featuring some islands of slightly more advanced vocabulary in a sea of very good basic vocabulary and grammar. Expect that your students will improve their speaking with the basics and improve their listening and reading with the slightly more advanced vocabulary.