First, never vouch for something you’ve never tried. If the product sucks, people will think the same about you. Second, only vouch for products you’ve used and actually benefitted from. Your credibility suffers if you’re vouching for a product that’s worthless. So, vouch only for products that have helped you. Third, it’s easier to drive traffic to your site if you have quality testimonials.
Once you’ve found a product you’ve been able to use, send the company an email. What you’re looking for is an explanation of how their product(s) helped you use your product(s). For example, “Dear Sam, I bought your XYZ and it helped me in a lot of ways. I was having a hard time marketing my website, so I started using your product. I ended up with 25 new sales in a short amount of time. But it didn’t stop there. Thanks to YXZ, I’m having the best month of my life.”
We've had more than 500 new sales thanks to your product! Thank you so much! If you're serious about building your business, you need to get this product. Now that you've explained your business through your link and how the product helped you, you'll get your link up on their webpage. You'll also get a one way link (which is good for your SEO). So, it should be easier to drive traffic to your site through testimonials.