How to make ourselves more visible to other aircraft?
Transponders, ADS-B (which, honestly, is pretty much just an outgrowth of transponders) and exterior lights have already been mentioned. I'd like to add one more thing: speak up!

You've probably got a microphone right in front of your mouth, which is ultimately connected to a radio transmitter which is hopefully tuned to a frequency monitored by nearby aircraft.

To help make yourself more visible to other aircraft, press the transmit button and give your location and intentions, to let them know at least one place where they should look.

As a real-world example, another pilot did this the other day as I was in the air, saying something like "XX Radio, SE-XXX on downwind 21, for touch and go". My instructor took the opportunity to quiz me about where they should be given that, and they were pretty much exactly where I expected them to be. Not only did we now know where they were (position and, by inferrence, approximate altitude), which we could have known by looking for other traffic, but we also knew what they were planning to do. In this case, there was no risk of a traffic conflict between us, but you never know who else is out there.

Unless the traffic situation is congested, or there's something like emergency traffic going on, the occasional radio call with your position and intentions can do little but help others.