It happens with everyone, when you start blogging, you do not use your own domain or self hosted wordpress blog, but after a few months or year you register a domain for you. And after you consider only your current website, and your old blog losses focus. But you can remain active that blog and you can use that blog to promote your current blogsite..
1) Ask people to link your website to their homepage and link them to your old blog, if they are agree.
2) Use banners of your site on your old blog to promote your site.
3) Link Your website on with your old blog.
4) Sometime right reviews for other bloggers to make relation with them.
5) Make it a general blog of any kind so it will be easy for you to post whenever you want and whatever you want. And some time post snippets from your websites.
6) Use customized Goolge search and add your website in when customizing, so when people will search on your blog they will redirected to your main website.
7) If you a no time to post anything to your old blog then use BuzzBoost to promote your websites content.
As your old blog is not your main blog so can experiment on it to promote new website.