No matter whether you're promoting an affiliate product or you're providing your own, you need to make sure you presell the product.
Preselling means to warm up your reader BEFORE he/she is sent to a sales page.
That means using good copywriting skills right within your email that will show people that the product that you're recommending is one that they actually need (or want). Empathize with the reader and show that this is a product that can do exactly what they need it to do.
That doesn't mean you're going to hype the product at all or lie. You can certainly make a lot of sales without making false claims about anything. It will be easier than you think to presell to your customers if you are promoting a product that you truly believe in.