Know best practices and make sure you adhere to them. Know what the worst practices are and avoid them. Know index internals. Know what makes for good database design. The tips here at the site can give you a lot of background. It's going to take a long time to master SQL Server so don't expect to be a master DBA in a matter of months.
Here are some starting points
http://www.mssqltips.com/tip.asp?tip=1652
http://www.mssqltips.com/tip.asp?tip=1707
