Unplggd offers a great little how-to for anyone hates mosquitoes (read: everyone) and is sick of buying new candles and cans of spray. All you need is some orange peel and a spent plug-in repellent you might have lying around.
Just cut an orange rind to about the size of a refill, stick it inside your device, and plug it in. It's easy and pretty intuitive, seeing as mosquitoes are naturally averse to citrus and citrus-y essential oils. Of course, you could always go the smelly sock route. [Unplggd]