I would say be well rested. this will reduce the amount of darkness under your eyes, if you have any present or don't have glasses to cover them up with.
One of the main factors to looking nice without makeup is what frames the face which is your hair. The hair can be kept nice and simple but tidy. The last thing anyone wants to do while trying to look nice is looking like they rolled out right out of bed. Also to keep in mind is to keep facial hair like eyebrows tamed and tidy as well.
Remember to drink your 8 glasses of water to keep your body hydrated
Eat more vegetables and fruits and less carbohydrates. This may give you a rosier and glowing face. Eat fruits that are high in antioxidants and vitamin c, so like berries and citrus fruits, to reduce sun-damage/aging/pigmentation caused by free-radials
Have a facial skin regime and always remember to wear moisturizer that suits your skin type. You don't want to your skin to be too oily or too dry. SPFs are better and abrupts any damage that maybe caused by the sun. Use a mild cleanser if you plan to use it everyday. I like to keep oxy clean pads which are quick and easy to removing dirt. Foundation is a barrier to dirt clogging pores so you need to take more precaution about any pimples or breakouts. many people use toners and exfoliators but i found that if i use them on a regular basis, my skin becomes very oily causing me to break out.
As others have mentioned.... workout!!! it will get your skin looking more taut and also boost your energy levels. If you want you can also give your self the one- man hand massage, it brings blood to the surface of your face making you look fresh. You can also achieve this by mechanical facial massagers as well.
Have confidence and believe that you look beautiful through your physical appearance and your personality. Another thing would be to smile most of the time. It doesnt have to be a full on show-off of your pearly whites but just a small smirk-ish or "smieyes" (smiling with your eyes)
you can also:
-Have your lips looking moisturized with chapstick or vaseline
-Curl your eyelashes with an eyelash curler
-splash cold water to look and feel refreshed anytime like during breaks
Great question, I see many people are wearing a lot of makeup every to university, I mean a lot. People wearing makeup to parties is understandable. I like to wear simple eyeliner in my waterline, cover-up for my hyper-pigmentation spots and chapstick.
less is more. :)