You should be able to determine, with most landing page packages, where most of your signups are coming from. It varies by business. If, for example, most of your signups come from one specific niche community... increase your presence there.

You can also determine a good source by calculating the conversion rate for that source. You may find, for example, that 50% of people from your Google Adwords convert, but only 10% of your Facebook visitors convert.