I make about 400 EUR every month from Amazon referals without actively doing anything.
I own a comunity fueld site (hosting pictures) that receives about 70k visits (20k unique) per month. Its just a little hobby project that runs itself and has been unchanged for 2 or 3 years now. It's hard to find advertisers due to the sometimes offensive content, let alone making money from banner ads.
So I did the next logical thing: putting an invisible iframe on the site that loads Amazon with my referal id. Anyone who visits my site and decides to go shoppind on Amazon in the next few days, still has this referal id stored in a cookie. Note that I don't actually link to Amazon anywhere. Conversion is soley based on chance.
My conversion rate is about 0.5% and I've been doing this for about 18 month now.