Promote upgrade opportunities inside the app
Everyone knows to pitch at the marketing site. But the sell shouldn't stop there. If you have a tiered pricing plan (or a free version of your app), don't forget to call out upgrade opportunities from within the product.
Tell folks that you'll remove barriers if they upgrade. For example, in Basecamp you can't upload files if you have a free account. When someone tries to upload a file, we don't just turn them away. We explain why file uploading isn't available and encourage them to upgrade to the paid version and explain why that's a good idea. The same approach is used to encourage existing customers to upgrade to a higher level account when they max out their current plan.
Existing customers are your best bet for sales. Don't be shy about trying to get repeat business from people who already know and use your product.