Keep in mind when you get a listing in Diamond's Preview for a particular month (two months before your title ships), you are in essence a tiny phone book listing, especially since Marvel, DC, Image and Darkhorse take up 4/5 of the catalog.
A comic book store owner will usually only buy new titles if...
a) a customer requests it and if enough of them do, then he/she orders a few extra copies for the store.
b) The store owner heard about it from other means (convention, website and/or magazine reviews, award winner, etc...)
c) the Prieview listing has a spotlight (which mostly is because Diamond loves it and want to spred the word about it, or you can buy a bigger listing with art, etc...)
d) big name artist or writer's new title.
c) the title is from a publisher the owner usually follows/buys from.
Now... A customer (fan) will usually only buy new titles if...
a) big name artist or writer's new title. especially if the customer usually buys stuff from that artist.
b) heard about it from a conventon, website, magazine before hand.
c) the title is from a company the publisher usually buys from.
d) The store owner recomends it (see letter b above)
How do you achieve all those points so you can max your potential sales.
1. Know who your fan base is. Demographics, age, sex, scifi versus horror fan, etc...
2. Send free copies to reviewers months ahead of time.
3. Send free copies to store owners or a postcard, flyer or something else.
4. Do book signings at conventions, stores.
5. Build a website for your title. Do blogging, message boards
6. Diversify. If it is a horror title, look at other fan websites and/or magazines for movies, novels, etc..
Advertising is more than putting a listing in Diamond or even putting a full page ad in CGB or having a stellar review from the reviewer at Joe'scomicnews.com. Effective advertising is a campaign that will cost as little as a phone call or as much as a billboard on Holywood blvd and any combination of elements in between.
The important thing is to not expect that everyone will buy it even though they know nothing about it.