Swap/Giveaway: This is a great place to start, call up friends/family and ask if they have already tried the product(s) and if so if they want it or are interested in trying it out if they haven't already. While doing this you may also want to ask them if they have any products that they also want to unload.
Bring It Back: I learned this the hard way. Don't buy products you have never used and let it sit around your home for weeks or months before trying it out. Keep your receipt and use the product within a few weeks of purchasing it. Usually it takes me one to two tries to know if the product is a keeper. If you don't like it bring it back for a refund or exchange, in these economic times every penny counts.
Sell It: Their are many resources available to sell products. If you have a Fotki journal you may want to post in your profile or in your product folder that you are selling a product. Also, you can post this information on natural hair forums and on Hairlista in their Buy/Sell/Swap section. Keep in mind that it is easier to meet up with someone who lives in the same city or surrounding area as yourself, preferrably in a public place, than to ship a product out of province or country. This will save on shipping charges.
Mix It Up: If the product you bought is not affecting the health of your hair, texture and look of it but you just dont like it for whatever reason you may want to mix it with other products. For example you can mix an oil you no longer like with your conditioner when you do deep treatments or you may mix it with other oils to do scalp messages or hot oil treatments.
Throw It Out: This should be the last option. Sometimes product junkism gets out of hand and you may feel overwhelmed with what you accumulated looking for that perfect conditioner, moisturizer, oil, etc. but when you throw out a product it should be because the product is damaging your hair/scalp and most if not all the above options are not available. I have thrown out a few products because of damage and I was afraid that giving it away to someone else would result in them having the same experience I had. It is better to loose some money than to continue using a product that causes damage to your hair/scalp. Think about all the products you can avoid purchasing to 'fix' your hair by simply not using it any longer.