I retail loads and to to be honest it's not a selling thing, it's just passing on information. For example, with the cold winter we have had everyone is suffering from drier skin than normal. All you have to do is either explain to your clients about your products, or even better give them a free demo ... on one hand only. Exfoliate their hand and then apply a moisturiser, the difference is amazing and they will be hooked. when it comes to paying just remind them, "shall I add the products to your bill?" Sometimes they say yes and sometime no, but if you don't ask you don't get lol!
remember as well to explain the features and benefits ie this has abc in which is great because it does xyz.
Add a few products every couple of months so you have something new to offer and so they will add to their collection, and you get to know what's hot and what's not with your clients! If you work in a salon have a well stocked display that entices them, and rotate it regularly.