The best questions are the no logical questions, ex; never use math questions. Math question can be resolved by many bots. Use questions like the color of the ocean is: blue, white o yellow.
Use many questions, +4
Change the questions every week, but it depende of your site popularity.
And there are two types of Sploggers, bots and "Chinese farms". With the first one, the security questions works great, but with the second one, only works the WangGuard algorithm. We are working hard improving the algorithm but when the "Chinese farm" use a clean IP, email or domain, is very difficult to stop them on registration. Normally, they are cached with the WangGuard cron after registration.