The purpose of this question is to collect a list of Tips and Tricks for using MO in a more efficient or otherwise better way.
If you know some trick or have a tip you want to share with your MO-colleagues, just add an answer here.
Please, only one tip or trick per answer. If you have more than one to share, just add a new answer.
Many will still remember the Tips and Tricks page from the old MO. This is (at least presently) gone, but even if it were to be restored, its content would need to be updated. A copy of this page was provided by Kaveh on the question of KConrad asking about the whereabouts of said page. There the idea for this question came up to organize the updating and (at least intermitent) displaying of this information. To just take an entry from the old list and to update it as needed (in some cases, perhaps not at all) and to copy it here as an answer is explcitly encouraged.