To avoid your customers’ dissatisfaction because of stock shortages, here's a solution that will help you save a lot of time while costing you next to nothing - automation. 
Once you've gone through the steps to get your warehouse up and running and chosen your inventory rule, you can pass on a few tasks to your system…

The aim of automated functions is to getting both hard and easy tasks done without your constant attention. The function is based upon stock levels you've entered, with a minimum amount at which the system must place a new order, and a maximum amount which represents the limit the system can’t exceed. Each time an item reaches the minimum level, the system will order it for you. It's as simple as that!

The more information you input into the system, the less you have to take care of. For instance, if an item is unavailable from your usual supplier, the system will automatically cancel the order and send a new order to the next supplier you have entered for that particular item.
You can also win on both time and money by configuring the system to hunt for best deals. The system will send offer requests to several suppliers for the same item and order from the one with the best price or terms, depending on how you want to operate.

If you own multiple warehouses, set the system in a way that you don’t get an overload of the same item. When necessary, the system will transfer items from one warehouse to the other to even up the amount.

Your stock might not just be things you sell - some pieces you hold in stock can also be items you need to run your company, and others you use within a production line. Forecasting your production will help you setup rules for your own supplies, so that whenever the time comes to get the production running again, you will not be held up by missing items.

Even if you don't have your own warehouse but instead use a method where you don't hold stock, you might want to consider automated functions.

For instance, in the case of drop-shipping, when a customer places an order from you, you then have to arrange for the item to your customer direct from a separate supplier. If the system orders directly from your suppliers, you get double the advantage: first of all, the delivery to your customer will be much quicker as it will only take seconds for your customer's order to be forwarded to your supplier. And you've gained precious time to deal with other tasks  such as growing your business or boosting your reputation with good customer service.