More RAM. It's the only thing that can reliably prevent it. Magic SysRq key[1] can trigger the OOM manually, which can sometimes rescue the system if you notice it soon enough. But if you run into the situation regularly under normal use, not programs running amok, more RAM is a must.

If you can't upgrade the RAM, zram[2] is also worth a try.

[1] https://en.wikipedia.org/wiki/Magic_SysRq_key

[2] https://en.wikipedia.org/wiki/Zram