I've been using the Prince Lionheart 2 Stage Car seat protector for the last few years now and it does the job at a very reasonable price of $20. Protects the leather, would not leave indentations on the seat and catches the nasty spills. You can get them at Babies R Us and other popular retailers like Target etc.
http://www.princelionheart.com/site_irl/t_cs_0560.html

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2007 E70 3.0