Good HID lights turn night into day (which I always switch off if possible once an animal has been sighted if speed/moonlight conditions allow), plus knowing how to scan the road properly and always ask a local about the area ahead if possible are the best ways to reduce your impact chances. A simple tip I learnt from a Pro Roo Shooter mate is that the little buggers won't move around on a windy night, so unless they're sitting on the roadside in a drought affected area, eating the greener grass or waiting for rain to fall or damp night dew to form on bitumen surfaces, you're less likely to encounter them if it's blowing hard. I'm guessing it's because their coats aren't very thick and they have no body fat, so they need to shelter to conserve heat.
The plastic whistle things would be more effective if you shoved them up your arse - the sudden rush of air through them just before impact might create enough sound to scare a stray animal into flight while you're hard on the brakes. Somebody got rich on the idea anyway. I guess that's what matters.