Back in the days when tickets were torn, I would always pre-fold my ticket ahead of time so that it got a neat tear along the dotted line.
Now that tickets are scanned, when I go to games I put them in a hard plastic holder and leave them there. They can still be scanned through the plastic, and the ticket remains in mint condition.
(Photo taken July 16, 2013. © Gary Dunaier. Link to upload on Flickr.com: here.)