Be very confident, but not cocky. In medical world, being confident is respected more than being correct.
Give the appearance whenever possible that you know what you are doing.
If you don't know what you are doing, appear very eager to learn and thank the person afterward for teaching you something.
Always think of the next step. In patient care. During rounds. For scut work for the day. This is hard to do, but really helps the residents and makes you look forward-thinking, organized and very motivated.
Come prepared, with the right equipment, dressed properly for the particular rotation/clinical environment. On surgery, wear your stethoscope in your pocket, not around your neck, and carry a pen-light, sterile swabs, 4x4, 2x2, scissors, suture removal kit, gloves, and be prepared to hand them to the resident at the appropriate time during rounds.
Be nice and respectful to people, even people who piss you off. This includes all staff, nurses, patients, other med students, your team members, etc. Don't be a gunner; help your fellow classmates.
Ask the resident your first day if you can pick up 1-2 patients. This will make you look like you take initiative and are eager to learn and help the team. Pre-round on them and give an organized and succinct presentation in SOAP format. Try to get through the subjective and objective parts fairly rapidly (don't waste time stuttering), so that you can focus on your stab at the assessment and plan. Even if you are completely wrong about the A/P, it is better to do it and say it confidently. The residents will usually kindly correct you, but they will respect the fact that you are thinking ahead, that you care about the patients, and that you were confident.
Although the subjective evaluation process will no doubt contribute to you becoming more jaded, try to limit this effect and realize that the most important thing is to learn. The more you worry about evals, the more they tend to bite you!
If focused on learning, helping and being respectful, you will do fine.
Hope this helps! 🙂