People within one industry are likely to want something that is similar to the products you are promoting.

You can think in terms of yourself. You buy lots of stuff, right? Consumers are all the same. People tend to buy products that are relevant to one another, though not exact. That is also the beauty of email marketing. One person will not buy just one product. Often times you will create a list of rabid buyers that will buy two, three or even ten different things from you over time. This is part of the power of the list and the more cross-promotions you can come up with, the better.

Let's look at an example niche "outdoor barbequeing". We entered the super broad keyword "barbeque" into the Google Keyword Tool . These following are the results that the search yielded.

People that are interested in how to barbeque, are likely also interested in many other topics. Since you have the ability to promote more than one item to an email list, it is good to build a list of ideas of things that you could promote (without being too off topic).

In this case, a broad search helped us come up with some great ideas. People that are interested in barbequing (cooking on the barbeque), would also be interested in:

(1) Barbeque Sauces
(2) Barbeque Smokers

(3) Barbeque Recipes

(4) Barbeque Accessories

You can also apply some rational thinking to this question.

"If someone were interested in barbequing, they would also be intersted in ________.

Off the top of our heads we can easily come up with a few things:

(1) Outdoor, nice weather activities

(2) The actual barbeque

(3) "Man" stuff, gadgets, tools, etc

(4) Cooking classes

(5) BBQ tools

(6) BBQ conversion kits

(7) Patio furniture

(8) Indoor Grilling

That is 11 different cross-promotions without giving this industry too much thought. With a bit of research you could have 20 different items that you could promote to an email list in this seemingly small "niche". Then you set up these promotions within your automated "follow-up" sequences, and you have a system in place where you are selling multiple items to the exact same person.

You need to remember, people are consumers and they buy stuff on a regular basis. If you can deliver to them ideas that are in line with their hobbies or things they would need, you are going to earn a lot of money.

Record your cross-promotional research!