The tool 'powertop' is very useful here, it will show what is causing any form of wakeup on your system, and offers a 'Tunables' tab with suggested settings to improve some aspects.
Powertop is great. You can even run it in non-interactive mode. That said, it only optimizes for lower power usage and lower interrupts. You can further reduce interrupts by disabling devices. If there are devices you don't need, you can blacklist them in /etc/modprobe.d/ provided you understand the potential dependencies other devices may have on the drivers. This can free up a tiny bit of memory as well.