I haven't had any interviews for SLP grad school, but for my prior master's, I made sure to:
Project confidence- look up power poses (or positive posture for sitting and standing), firm handshakes, eye contact
Have a good answer for 'What are your weaknesses?' and 'What challenges do you anticipate?' It should be truthful, but also not harm their perception of you.
Practice stalling for time without sounding foolish. This can be through rephrasing the question to make sure you understand correctly or breaking the question down into smaller components and then answering them.