It is important to not fall into the trap of focusing all your attention on the traffic sources that are proven to work. You need to save some time to also try new places to advertise to reach new audiences.
As you may know, I have been promoting the different launches and new lead capture pages over the last year and have been successful, adding several hundred referrals to my list each month. This is a good example to illustrate the need for new traffic sources.
When a new and compelling program first launches, almost all advertising sources work. There is a big demand for people to join the program and almost no one has joined yet, so everyone is a prospective signup.
However, as more people join and start promoting, it gets harder and harder to find new signups because you are competing with more advertisers and a lot of the people viewing your ads have already joined the new program.
This creates a problem called saturation. The traffic sources you are using are too saturated with people promoting the program and most of the easy signups are gone.
However, if you have multiple traffic sources, and aren't reliant on just the most popular ones, then you can keep your conversions high by focusing on the less used sources.
This doesn't always mean finding new sites to promote at. Sometimes you just need to use all the different ways that the sites you already belong to have to reach their audience.
Are you using solo ads from your existing sites? How about login ads? Banners?
Most of the sites you belong to have multiple ways to get your ad in front of their members. If you are only using one of them, then you are missing out on signups. Also, since most members don't use all these other ways to promote, they are less saturated than just the safelist emails.