... but they depend on what you're selling. At a minimum, we can think of four very basic revenue models:

If you sell a product like a t-shirt or computer program, you could either sell it as:

• A single purchase, where the customer "owns" the product forever (like most t-shirts), or

• A subscription, where the customer has access to the product for a fixed period of time (like a subscription to Microsoft Office which you pay every year)

If you sell services like mowing lawns or consulting, you could either sell them as:

• Time and materials: you bill the customer afterwards for the actual hours and purchases needed to complete a project. This makes sense when the requirements are vague, or there is a lot of business risk to you.

• Firm fixed price: you bill the customer afterwards for a fixed price and scope agreed upon before the project began. This works best when you have a great understanding of the needs, what it takes to get there, and can build in a profit buffer that your competitors can't.

Once you've figured out the right revenue model for your business, you need to think through how and when you get paid. Here's a few models and things to think about:

• Pre-payment: customers pay you in advance, and you perform the service or delivery later. This is exactly how you order clothing online when you pay at checkout, and (hopefully) your order is delivered a few days later; when you pay a deposit to hold your desired dates at a venue; or when you purchase a gift card. For the entrepreneur, this is great: you get the cash you need quickly and don't need to worry about collecting payments from customers later. However, not all customers (especially large ones or strategic purchases) will accept this.

• Extending payment or credit terms: sometimes, you may deliver an item or service, then invoice the customer afterwards. Typically, you would allow the customer 30 days to make the payment. In this scenario, you need more cash to hold your business over, which will cover the time between paying your suppliers for inventory and getting the cash from your customers. While this model is necessary for many businesses, it requires cash to grow (we have tips on how to help), and there's a risk your customers will not pay for the goods you've already delivered, which we call "bad debt."

Which model works for your business depends on what is accepted in your industry, and more importantly, by your customers. Just make sure you take the time to understand the cash flow implications of your choice as you grow.

Finding new customers and keeping your current customers coming back for more drives the revenue for your business, although the dynamics look a little different depending on your revenue model:

• Single purchase products: New sales are important, but you can also cross-sell additional items to existing customers by nurturing them

• Subscription products: New sales are only half of the equation - real value is created when your customers come back and keep renewing. For subscriptions, increasing renewal rates is extremely important.

• Projects and services: Regardless of whether you sell these as firm-fixed price or time and materials, your growth can come either from new customers, or from cross-selling additional (or repeat) services to existing customers.

In general, retaining existing customers is usually a faster way to profitability than obtaining new customers. However, when you are just getting started, building and diversifying your customer base is important.

Check out our Right Growth tool to find the targeted advice that fits your challenge and your business.