Bootloaders can also be used. But to go down that route you have to tune the internal oscillator and hope that it doesn't detune itself.
I modified a version of Optiboot to work properly with Attiny devices. The bootloader includes CodingBadly's TinyTuner sketch to allow you to tune the oscillator and have the tuned value be loaded automatically as part of the bootloader routines. Once the oscillator is tuned, the tinyTuner sketch code is deleted meaning the bootloader takes up only 576 bytes of flash.
The bootloaders can be found here: