Psychologically, losing things is twice as powerful as gaining things so it stands to reason that we should motivate change through fear of loss rather than through desire to gain. As this Wiki page argues, "Humans may be hardwired to be loss averse... the loss of a day's food could cause death, whereas the gain of an extra day's food would not cause an extra day of life".

From a sales point of view, this means that getting a client invested in your product or service, whether financially or through spending time to create some type of bond, will increase the probability of future sales in a manner that is disproportionate to any discounts or future-gain incentives you might offer. 

This would suggest that combining any sort of trial period which requires input from the client will improve the probability and size of an initial sale. The trade-off being that the sales-cycle will need to be longer to incorporate it.