Provide frequent opportunities for feedback. Students need to understand how they are doing in relation to their peers, to your expectations, and to their final grade. Instead of having students wait until a midterm to gauge their performance, try giving short knowledge quizzes on previously-covered material. Clickers are a great way to do this.
Help students interpret the results of any assessment. This begins with simply reporting the mean and standard deviation for every assessment. Then let students know what is considered "doing well" in the class, and what they can do if their grade is below that (e.g., come to office hours, attend an extra lab, meet with your TA).
High performing students need to understand their performance as much as struggling students. Sometimes we are so focused on helping students in trouble, that we don’t give enough feedback to those who are succeeding. Don’t hesitate to drop an email to a student who is doing well letting them know that you noticed. This kind of recognition from a professor can be transformative, especially to a student who may feel like they don’t belong.
Encourage students to seek help. When a student struggles, encourage them to persist through the task, and make it acceptable and easy to seek help. For example, periodically remind students when and where they can get help and genuinely encourage them to take advantage of it.