1. Getting more customers
2. Increasing the transaction size (i.e., getting customers to pay more)
3. Increasing the frequency of purchases (i.e., getting customers to buy more often)
Often increasing the transaction size is put forth as upsells or cross sells. Rarely mentioned is increasing price. I think that is one area to look at hard and do testing on. A membership site with 3,000 members only has to increase price by $1.00 a month for a ton more revenue. An extra $5.00 per product would make a big difference if you are selling thousands per year.