I think you might be looking at the problem the wrong way around. If you want to increase the number of voters, make the voting anonymous. There have been many a contest where I haven't voted, simply because that meant I had to publicly announce who my favorite was. In doing that I felt like I was taking sides in an argument and the people I didn't vote for would dislike me. PM voting would be an easy way to solve it.

Sadly. You will never come up with a system to defeat friend voting blocks. Even RL Politics hasn't figured that one out.