you should do a better by:
1)seo
2)post on all top forums, and free directories
3)talk with your users who are mostly active to promote by mouth, not only you your admin and modertors need to promote
4)also talk with softacoulus, fantastico to add abantecart,
5) Keep your support mail more promt because user need a quick response as other cart software has very dull support (Free one) you need to be take advantage of this
6) There are lots of news paper who provide free articles talk with them
7) Develop a better tutorials
be supportive always for your users
9) Be in partnership with freelancer developers and companies who develop eccommerce solutions
Hope this will help