I use a little Ambi-Pur thing which clips on to the passenger air vent. Even has a little plastic slider to adjust the smelliness.
Because it's the air coming through the vent which 'powers' it, there's no need to do anything to do once it's setup (apart from replace the little glass fragrance bottle every 3 months or do, when it's run out).
You can see them on Ambi-Pur's website, but it's all whizz-bang Flash based stuff and I couldn't see
how to link directly to the product page.