Here are some tips on e-commerce-

Encourage Impulse Buys: i.e. applying techniques used in regular shops and restaurants; e.g. in a fast food restaurant "would you like to supersize your meal?"
Suggest other items of interest: when a shopper clicks to view details of a certain product your detail page could offer a discount if the customer buys another similar product.
Remind customers of linked product: eg if the customer buys a dress, ask if she'd also like to buy a necklace, bracelet or other accessories.

Offer deals for online customers only: Many customers will browse your site to get an idea of what product they'd like but will then drive to the nearest store and make the purchase there. This is probably partly because people like a tangible product. However, you can provide an incentive for these shoppers to buy online. Offer them a deal only available at your website. Be careful though; make sure it actually is a deal. It is important to retain the trust of customers and most are educated enough to know what a rip off is.
Ship larger orders for free or for a discount: Shipping costs are one of the main reasons many shoppers will find the product they want online but hop in the car to buy it in a store. If you raise you profit margin but offer free shipping for orders over a certain price (or certain number of products) you will not only encourage more people to actually buy online, but also to buy more products than they initially intended.
Note: Make sure the customer can easily find out the shipping charges and options before they register or go to check out. Countless shoppers will look for this first and will leave the site immediately if they don't find anything.
Make sure your website is appealing to customers. Design is important is that is what will hit the shopper first, but what visitors will really be looking for is good content. It's worth paying to get a good copywriter to format your content to reflect how people read online and be persuasive and convincing.
Provide excellent customer service: This does wonders for you site traffic, reputation and profit. If you have impressive customer service shoppers will be far more willing to return to your site and buy more, and will also be inclined to recommend your site to others. It's definitely worthwhile as there is so much competition out there and yet not that many companies these days do have good customer service. Therefore it's a great way to make your site unique and stand out.
Make it easy for shoppers to buy from your site: It is beneficial to have an option to register as you will be more likely to keep customers coming back for more, and they can be sent details of the latest offers; but you're unlikely to get any customers if they have to register before they can do anything. Give them the option and many will register after making their first couple of purchases. Also you should have a site search that works better than most customers will expect. Make sure you have one that can find products by SKU, Model Number etc. and if possible: misspellings. This will impress customers and keep ones that would have left the site if they didn't find what they were looking for on the first try.
Don't try to do everything yourself: It is important to have many opinions on how your site is run; it is impossible for you to notice every little thing that could be improved. It is really useful to hire an e-commerce consultant and/or a copywriter as they will have the expertise knowledge to really get your site out there. If you're not sure about who to hire it is helpful if they are from the same area as you are based; for example I would search for "e-commerce consultant Glasgow [http://www.alisyme.co.uk/consultancy/e-commerce.aspx]".
There are countless more tips on e-commerce but these are some of the main and most important ones.