If a sales person is constantly missing their quota, and the rest of the sales force is beating it, then it is probably time for that sales person to shape up or ship out. If no one is beating the quota, then maybe it is a little high. You need to experiment with quotas to see what is right for you. You should also have an incentive for sales people to beat their quota. Sales people are competitive people, and contests work.
Some companies have a commission accelerator or bonuses for breaking quota.
Many companies have 'President's Club', which is usually a company sponsored trip for those sales people who beat their yearly quota. After you have determined your sales person's quota, the next fun step is to determine their commission percentage. The actual percentages may vary for your ISP, should be determined by your profit margin, and may be used to convince sales people to sell certain products over others. Both structures have plusses and minuses, but one essential factor will be your accounting and payroll system. Commissions are usually paid 1 month behind to assure the sale is real, and the customer was added and billed.