If you have many people that abandoned shopping cart during checkout process, there are a lot of reasons for that. One is that many people make decisions on buying stuff on the spot initially from emotional of point view and not from logical where someone will start counting positives and negatives on making a purchase. So when potential customers start the checkout process the duration of time and effort to complete data in the forms will have to be kept on minimum, the best way is to remove all the unnecessary friction.
Clear checkout page from distractions
Final total price
A clear view in final price is very important so every potential buyer would know exactly what he will pay indifferently of his choosing payment method and calculating total shipping cost or discounts from coupon codes accurately is very important. There are many cases that somebody use a coupon code and there is no proper integration with the payment method so when is redirected to payment provider in screen appears the toal price of cart without the discount. This pure quality integrations can prevent many users from completely the purchase and also lowers trust on the online store.
Customer support and refund policy
A clear return and refund policy will make more comfortable users to buy online and specially on items like clothes that you cannot have a clear idea on the quality from a picture. You could order a jacket that could have a good quality but was created from thin material and not suitable for winter, it has to be possible to return and taking your money back easily and without problems. Also in ecommerce business refund policy is obligatory by local laws.
Multiple payment methods
Having a wide variety of payment methods you do not allow the customer to think twice on how will pay. The best way to achieve is to have as many as possible available methods such as credit cards, cash on delivery, paypal or wire transfer. So do let customers think twice about that and if already existing customer in the checkout page list first his preferred method of payment.