I used Who Sees Ads plugin to display the signup box above the content. I made the settings in such a way that users who visit this blog through search engines will only see that signup box and regular visitor won’t. The posts which are 20 days old will also have that signup box above the content.
I’m not sure that how exactly this will boost the signups as I just implemented it, however I’m positive that this will surely boost the signups as the signup box is above the fold and right above the content. So, if you are not happy with those ugly looking and obtrusive way of increasing the signups, you can try this trick for sure.