High Converting Website
As Jeremy mentioned - Opt in Generator WITH Social Liking
Close 4th - Social Media Strategy and properly managed channels
Marketing is everything... but a lot depends on what kinds of products you sell... it is kind of a big open ended question. Not to say SEO isn't important... it is... so whenever you create your pages create them with clear titles and descriptions, etc... and it helps to use an SEO company to help create backlinks, unfortuantely there are a lot of terrible SEO companies out there.
These days social marketing is probably more important... checkout Pintrest, Polyvore and other social product sites.
#2... Focus on your business... everyone tries to do everything to save the most money, and that just doesn't work. You should have compaines or Apps hannding, SEO, Facebook mgt, Google Adwords, Google Shopping, Product Mgt and so on.
Your top 3 should prbably be
1) Google Shopping / Adwords
2) Social Media (FB, Instagram, Pinterest)
The majority of your traffic and sales will still come from Google Shopping / Adwords... it all just needs to be finely tuned. Depending on what you are selling, FB could generate a ton of sales, peticularly for trendy products. SEO you can spend a lot of time on, and cost, and get little tangible results.
We have an app that integrates with Shopify to assist you with Google Shopping, as well as market to other CSE's and Marketplaces such as Amazon, Sears, Shop, Pricefalls, Jet.com and more, as well as offer Distributor Management for easy product upload to your Shopify store and sync inventory and much more. Learn more at http://apps.shopify.com/product-feed-manager
Instagram is a given and having a blog is also a great source of traffic / brand reputation. But KickOffLabs was a great tool for me to caputre emails and launch a contest. 150 emails in 3 days captured... still counting.
I have two MUST HAVE tools that have made us over $10,000.
First is: Fishbowl Apps (https://apps.shopify.com/fishbowl-prizes)
This allows you to host a giveaway on a marketplace, a simple offering of a $50 prize over 7 days got us 2500 emails.
Second is: Rare.io (https://apps.shopify.com/rare-io)
This is free for us because we don't have 20,000 emails yet but it has a nice drag and drop email builder along with analytics of our customer base. It also sends each customer a different email showing the product they're most likely to purchase next arriving in their inbox when they're most likely to shop on your store.
The combination of the two is perfect, one gets you emails the other gets them to buy!