[thomashobohm]

In terms of aptitude testing, going through some books like "How to Prove It" and working on rigorous proofs-based material (perhaps an introductory analysis book with lots of diagrams) will be helpful. As will brushing up on geometry-lots of people memorize a lot of geometry in middle school/high school, do well in their class, and forget it. Geometric concepts continue to appear throughout math and having a deep grasp of them is super helpful for your first "rigorous" math classes, where the secret to finishing a proof often lies in visualizing or drawing it correctly.