I've recently begun to learn the ropes as an Amazon reviewer. I am by no means a pro, but it has been a fun and enjoyable journey so far.  I'd like to share a little of what I have learned so far.
     The most important thing is to have Amazon Prime, in order to take advantage of the free 2day shipping.  Always read the listings carefully to make sure the product is eligible for Prime and not going to charge shipping prices.
    Many of the items I have gotten through the various review programs I have tried are free or deeply discounted.  This does require a disclosure stating that you got the item free or discounted in exchange for an unbiased review.  Another important thing is to have a nice bio on Amazon kind of stating your interests and stuff,so that sellers know you are a real person.  While you're on Amazon, review everything you've ever bought to help show sellers that you will follow through with posting reviews.
     The review programs I have tried are free to sign up.  You won't be approved every time for every item because it is up to the individual seller to decide who they want to offer the promotions to.  I have gotten a wide variety of products, from phone accessories,Bluetooth headphones and speakers, nutritional supplements, fitness products, kitchen stuff, even essential oils and diffusers just to name a few things off the top of my head.  All free or deeply discounted.  Each program has different requirements and you will want to do your research before you get products.  I started this out of boredom and now I am hooked!
     Now I'll list the review sites that I have had results with.  Product Elf is a good one and pretty simple,they add new things a few times a day.  My favorite is Amzreviewtrader.com and it is a little more challenging but totally worth it and they have a wide variety of products that you can browse through and request to be considered for the review.  Jungle Launch is also easy, they will email you when a campaign is available that may interest you.  Elite Deals Club sends out daily emails and if you can get on there when it is time for the deals to go live, it can be really simple to get a few more products through their program.  I have not had any luck with Tomoson yet but I am still trying.
    I take my reviews seriously and am trying very hard to be a thorough and reliable member of the Amazon community.  It is a learning process and I do stress that you need to make sure you can fulfill the obligations for your reviews and the programs you got them through.  It can be frustrating when you don't get a lot of approvals but keep trying.   I am constantly trying to get better and luckily for me, I'm entirely too opinionated anyway so I usually have plenty to say.  I've discovered some really great companies and products on this little adventure.