By improving your lookout technique, by using situational awareness, by being aware of where aircraft may be hiding, by manoeuvring to take a look at all those "hidden" areas.
yes yes yes, it is so easy in your black and white world.
So you are on an IFR flight plan in VMC, you have been given a descent and are under radar control. You are complying with ATC instructions, looking out, scanning with two sets of eyes onboard, there is nothing to see unless you start Lazy-8ing your way around the sky (which might p*ss off the controller). Suddenly "traffic alert, pop up traffic right below you, climbing, stop descent now, turn right 270 expedite"........
Now imagine the same scenario (which happened to me) with TIS onboard. You'd get a traffic alert showing the aircraft position on the screen at the same time as the controller saw it, adding an extra level of information. Even without the controller, disaster could be averted.
Now imagine it with no "technology" to help....