How do I properly tighten my seat?

In almost all cases, the nuts need to be tightened even more.
 
Although the seat nuts may already seem quite tight, they will need to be even tighter-- as tight as you can get them with a full size wrench. Make sure to tighten the two nuts evenly. This is very important.
 
One more important detail: the seat clamp must be pushed all the way down onto the seat post. If the clamp is not fully engaged, then the seat will never be secure, regardless of how tight the nuts are.
 
I will include a video below created by SixThreeZero, that explains how to tighten the seat using a wrench and ratchet. Though the video is for an outdoor bike, the concept is the same.

https://www.youtube.com/watch?v=WXRD0o2r0XU