Put it away in its box and don't use it!
Realistically that's the only way of stopping your Amiga failing.
I filled up the IBM 320Mb HD on my A1200 with loads of great software and then put it and a Microvitec monitor to rest.
In the meantime I'm using emulation to keep me going, and my true A1200 only when I feel overwhelmed by nostalgia.
The fact is the more hours you leave your computer on, the greater chance of mechanical failure from the drives.
However, the greater number times you turn your computer on and off, the more stress you exert on electrical components due to heat expansion and contraction.
Conclusion - don't use it! Or at least use it as little as possible.