There are multiple ways to improve your delivery/open rates. There is not just one thing that you can do but there are a variety of factors that may help.
The first part of doing a suggest campaign is using an effective subject line. We suggest doing multiple tests in chunks to see which subject line is the most effective. For example, if you had a list of 100,000 you can send 50,000 to the first subject line and 50,000 to the next subject line and see which subject line is the most effective.
Another helpful tip is for campaigns is to turn on the "view as webpage" feature. This will help increase open rates since many of the email providers will disable images that stop view tracking by default and instead the user is taken to a webpage that contains the whole message with the images loaded.
Keep your message clear and simple. Avoid all caps in message and subject line. Avoid keywords that might trigger spam filters. We have a built spam-check system based on spam assassin that we suggest to run against your message and subject line before sending out your campaign.
Be careful about how many times you mail a user. The more often you mail the same recipient that might unsubscribe or worse report the message as spam.
Check your list quality. If you have a lot of addresses that do not exist or server not found that may indicate a poor/old list. The quicker you can remove the inactive addresses the better your delivery will be over time.
If you have multiple IP addresses we suggest to rotate them by mailing using one IP address one week then the next week use a different IP address which allows the first IP to cool down for a while.
We suggest changing out a domain every couple of months if you are a high volume mailer to help improve delivery rates.
Some tips that we have to improve the results/delivery.
Setting under "create campaign advanced settings":
Domain Delivery Groups - Limits the number of connections to popular isp's and is the known/best practices for sending to those isp's
Sending speed if time is not a factor send as slow as possible on new IP's/new data. Check the FAQ for details on sending speeds. Generally, the slower you send the better the delivery you can spread it out across multiple IP addresses. Over time once your reputation is built up you can increase the speed as well.
DKIM - turn on DKIM if you are sending to consumer emails such as yahoo.com,gmail.com,comcast.net those providers look for DKIM and will give your mail a better rating.
General tips. Run your message against the spam check score system and try to fix any issues that are discovered.
Turn on reputation monitoring for your IP's and domains. It can be found under IP addresses and domains in the sidebar. This will auto scan the IP's and domains every so often and check against over 100+ blacklists and for the IP addresses get the sender score for the IP.
If you are using purchased lists or data that has not been clean. Dedicate an IP address or server to just cleaning the data and after sending through a couple of times move that data to your account.
Setup feedback loops for the major isp's we have quick links that can be used to set them up. This will reduce your complaint rate from them and help improve your reputation.
There is also an option to include the "list unsubscribe" header under system settings which will make it easier for people to unsubscribe instead of reporting to spam and levels protect reputation.