Your best practice for in house reviews, although I cannot say it is the most ethical, would be to create customer accounts and write reviews on the products. Make sure to spread these out over time with different usernames. As far as google goes, if they're all coming from the same IP address, they might not be so helpful but using the keywords anywhere on your page in text is always good!
When customers see these good reviews they will feel better about making a purchase and that helps conversions(which is the reason you're doing SEO in the first place).
Another way to do in house reviews is to do them on the company blog. This is a more white-hat approach and you can link to other pages on your site where the products are.
The best way to help your page though is to ask your customers to write reviews. Offer them incentives(like a 5% discount on their next purchase when they review the product they bought).
Having real reviews on your site is definitely a great way to give your site a better reputation. Hope that helps!