1. A handy app like Flashify or Rashr
2. If you’re a franco.Kernel user & have FKU installed, You can use the built-in File Manager to browse to anywhere on your SD card & choose your boot.img
3. Via adb:  fastboot flash boot boot.img
4. One can also temporarily flash a boot.img into memory that only persists until the next reboot whereupon the the original permanent kernel is used again: fastboot boot boot.img
5. Open an existing flashable ZIP, replace the boot.img with the newer one then flash it via recovery
6. Copy your boot.img to the root of your SD card. Then type the following in an adb shell:  dd if=/sdcard/boot.img of=/dev/block/platform/msm_sdcc.1/by-name/boot.img

— credit goes to Francisco Franco (XDA, G+) for inspiring me to write this