There are people who buy in the moment and others who will think about it and come back another time.
The problem is the second group of people (who come back later) may forget to come back, may not find your site again, or may end up at another site and purchase there.
If you provide a final date for a promoation or a sale, you create a sense of urgency for the people who would otherwise leave and come back some other time.
This concept is simple, but he results are usually very substantial. People like deals and people don't like losing out on deals. So creating a deal which is going away triggers both senses and results in more purchases.
Many online buyers don't trust testimonials since it's hard to verify their authenticitiy.
However, when you include information that helps identify a person alongside a testimonial, it makes their recommendation of your products and services much stronger.
You can go further and provide some kind of contact information (like a Twitter id), so the visitor can verify the source of testimonial themself.
When somebody is on the fence, the offer of money-back guarantee can give them a nudge and ensures that they will be satisified either way – either you product/service will give them what they want, or they'll get their money back.
This is effectively a promise between you and the customer.
And like all promises, a few things to remember:
At every step of the checkout process, the customer could get distracted and leave or change their mind. Then you end up with a lost sale.
Once the customer has "almost purchased" you want to do whatever you need to in order to smooth the purchase process. The less steps the user has to go through, the more likely they'll completely the transaction successfully.
Amazon has perfected this with the one-click checkout option.
Some examples you can do:
As a person interacts with your company, they'll go through a number of touchpoints.
They'll read your blog posts.
They'll interact with you on social media.
They'll check out your online store.
They'll read your about page.
They'll subscribe to your mailing list.
They'll fill in a survey you sent them.
.. and so on.
With each interaction, you have the opportunity to learn more about the customer and send them targetted promotional offers.
For example, if someone has already subscribed to your mailing list and is then checking out your online store, offering them a time-limited discount might be all it takes to convert their curiosity into a sale.