As part of an improvement process (any process, not just database development), you can start by asking yourself “How could I make this process as bad as possible”. In fact, when you think of how to royally screw something up, all you have to do is flip the answer around, and there you have a good starting point for improving your process, work, whatever.
Here are a few T-SQL “worst practices” to get you started on improving your database design and development practices. I’ve personally seen all of these in the field, and I think you might recognize some of them, too.