You'll need for this one to be as closely related to your website's theme as possible. The first reason is because you don't want to turn your visitors away by having them thinking they've come to the wrong site. The second reason is because you want it to appeal to the majority of your already targeted website visitors. You're in trouble. Putting a subscribe box on every page of my website immediately boosted my subscription rate by almost twice as much.
Every little bit helps.
Now, I get about 30 subscribers a day from the site subscribe forms, my home page subscribe forms, and subscribe forms on my partner websites. Well, that's reserved exclusively for subscribers of The On-line Marketing Diary.