1) Add it to the side bar... Works well for blogs...
2) Add it to the header or footer... For extra impact you could get it to slide in on the page after a few seconds...
3) You could consider a pop-up either when they arrive or on exit... Can be annoying so consider using cookies to check if it is a returning visitor with not showing it again.
4) You could create a landing page and place a menu item "newsletter" or "free bonus" etc...
It all depends on the type of website you have and the style that you want to maintain.... You can go for one or a combination of the above... I would go for it and over time experiment with different approaches with split testing....
On that note what do you have to wait? I hope you get round to implementing it today!!! ;-)
I would say, on the top right of your website!
A lot of Marketers have Wordpress websites, and a opt-in form would be the first widget @ the top.
Hope this helps.. :salute: