This may sound so simple, but perhaps was something you didn't even realize!
Have you ever called one of those customer service numbers you find on products? Emailed a company to compliment or complain about a product of theirs? Well I highly suggest doing this!
Often times, the company will send you valuable coupons, or even coupons for free items! In the past, I've received many high value coupons or free items by writing to a manufacturer (usually to compliment).
Some that I've received include: Glade, True North snacks, Iams, Pepsi, Oregon Chai, Bayer, Digiornio and more!
One time, my boyfriend bought a 6 pack of beer. Upon opening the first one, he noticed a little skunky taste. Well about 2 beers into the 6 pack, he decided to call the customer service line (and let me assure you, the conversation got funnier as the 6 pack went down!). They wanted to know all kinds of details about where he purchased the beer and numbers from the bottle. In the end, they sent him a coupon for a FREE 6 pack AND a prepaid box to send back a sample of the "skunky" beer!
I suggest praising or complaining about a product only if you have a legitimate things to say. They'll know if you are just sending generic emails, trying to score free products. Include something specific, about how it reminds you of your childhood or makes your life easier in some way - companies love this!
The blog, Couponing to Disney, has a "5 a day" campaign, where she suggests 5 different companies each day to email. She also has a great comprehensive list of companies she has already contacted, and what she's already received from them. You can find that here.