First focus on changes that, if they worked, would result in a 5–10x improvement in a key metric. This could be something like an entirely new email auto-responder sequence, a new Web site design, or a new onboarding flow. Once you’ve made big changes, then optimize the smaller stuff.

Your goal in designing these invitations is to get potential customers to engage with the invite and follow the link (or take the next step) that the invitation contains. Invitations that work best are short and succinct. Sign up for the most viral services you can think of and you’ll see what we mean. Additionally, personal hooks really help.

Shortening your viral cycle time drastically increases the rate at which you go viral, and is one of the first things you should focus on improving if using this channel. To shorten it, create urgency or incentivize customers to move through your viral loops. Additionally, make every step in your funnel as simple as possible to increase the number

Other products grow by encouraging collaboration. In this case, the product is still valuable on its own, but becomes more so as you invite others.

Last, an effective email sequence will be meaningless if you don’t have great email copy. Copywriting is an art on its own, but we suggest checking out some of the resources and information that Copy Hackers provides. An email campaign can easily go from a waste of time to wildly profitable just by tweaking a few words and headlines.

One of email’s strengths is that it’s a way to get feedback from your customers. One trick Colin told us about was not to send any email that comes from a “Noreply” email address (e.g., noreply@facebook.com). Instead, use that opportunity to send the automated email from a personal address and allow the recipient to reply with questions or problems

Effective email campaigns A/B test every aspect: subjects, formats, images, timing, and more. Timing is especially relevant to get higher open rates: many marketers suggest sending emails between nine a.m. and twelve p.m. in your customer’s time zone or scheduling emails to reach them at the time they registered for your email list (e.g., for

For these emails, you should determine the steps absolutely necessary to get value from your product. Then create targeted emails to make sure people complete those steps. For those who fail to complete step one, create a message that automatically emails them when they’ve dropped off. Repeat this at every step where people could quit, and you will

consider advertising on email newsletters complementary to your product. Many email newsletters accept advertisers, and if not, you can contact them directly and ask for a special arrangement.