This is no longer the 1930's where we waft along at glacial speed, in largely empty airspace waving casually as someone flies past.
Modern solutions are needed to deal with congested airspace and the wide variety of machinery trying to use it.
The eyeball is important, but solely relying on it is a flawed for the reasons stated in previous posts.
Accurate flying and concise communication about intentions will also help others take avoiding action before a conflict occurs, especially in uncontrolled airspace.
The place to start is using the tech that is available, it helps no one if it is turned off or unserviceable. Getting back to basics and being disciplined in uncontrolled airspace may also help.
On the TCAS front, Garmin's GTS800 is TCAS v1 and in it's cheapest form provides about 12nm of advanced warning. Equipment costs were about $10k excluding labour and paperwork. If you spend more you get better range but for most light machines that is probably sufficient.