If you want to sell more of a product or service, the first thing you have to do is discover why you're not selling enough already.
#1 Work Ethic:
Selling requires you to dedicate time to get better at it. You can't level up if you're putting in just enough work to get minimal results. If you're not working hard to consistently improve, you're probably not gonna make it in sales.
#2: Competency:
You have to know what you're selling. You need to have a good enough understanding to get others interested in the details of your offer. If you're not confident about selling the product your client or customers aren't going to be confident in buying it either.
#3: How-To:
If you don't know how to properly do something or relay information for a product, you just have to make the effort to learn more about it. You can read books about the subject, ask questions in relevant online communities...
#4: Identity:
Numbers 1-3 will help with this; the best way to confirm your identity is to create it through your environment. Become who you need to be to offer the level of confidence your offers require. Surround yourself with people who are way ahead of you-- become comfortable with the discomfort by forcing yourself above it.
#5: People Skills:
Size people up. Study the people around you and learn what they're thinking and expecting. Body language is huge and the easiest way to know what people are feeling. Pay attention to how they're reacting to you and change your approach so that it caters to their needs. Progress things at their pace and allow them to become comfortable with your offer.
#6: Belief in the Product:
If you don't believe in your product you can't expect anyone else to. Your enthusiasm creates enthusiasm for your clients. You have to be able to sell the excitement for what you're selling as well as the product itself.
You should always be improving every chance you get, day after day. The biggest thing holding you back is not knowing where to get the info and insight you need to do that.