Using a link renting system. Basically you buy credits and pay for links per month. This can work out costly but if you can find a secure system (not accessible to Google) that has good quality, high PR links then it is possible to use this as a highly effective method. Note it is very important that you have expert advice to get links which will work. Many people spend months attaining links which simply have little or no value without even realising this is the case.
Done right 9/10 - Done poorly 5/10