A few months ago, I began e-mailing my favorite food companies (along with a few others I heard were generous with freebies) with product compliments. I Google to find their website and then click on the contact us link. I write a brief message, telling them how much I like their product and maybe mention a specific product or reason why I enjoy it. That's all. E-mailing one company takes just a few minutes. I never request coupons, but MANY companies send them anyway, often ones for free products.
Here are some of the companies I've contacted, which have produced the best results.
Goody - Free package of hair bands
La Brea Bakery - Coupons for Free loaves of bread
Dannon - coupon for free yogurt and other various coupons
Chobani - coupons for free yogurt
Marzetti's - coupon for free product
Clif and Luna Bars - coupons for free bars
Glade - Free product coupon
Deer Park - Free case of water
Turkey Hill - multiple $1.00 off coupons
Arnold Bread - multiple $1.00 off coupons
Johnsonville - multiple $1.00 off coupons
I've also found that writing companies with a LEGITIMATE complaint usually results in a very quick fix, that is a voucher for a free product or a refund. I recently wrote to one company whose breast milk storage bags leaked (pretty upset about that one). I also wrote a yogurt company, whose yogurt was spoiled when I opened it, weeks before the expiration date (could also be grocery store's fault, who knows). Both companies were quick with an apology and a refund. With a valid complaint, I feel that it is a totally acceptable practice to contact the company to politely share your concern.
So, whether you have a critique or praise, consider contacting some companies, and enjoy the surprises that will start flooding your mailbox!