What to do if I cannot drop database because it is currently in use?

If you cannot drop database because it is currently in use - Make sure you don't have dependencies like database snapshots on the db you want to remove. Though, the error message would look otherwise. Are you sure that there is no hidden process that is connecting to your database? A good approach would be to run a script which kills all sessions and immediately after rename the database to another name and then drop database.

create a cursor based on this select:

  select  d.name , convert (smallint, req_spid) As spid

      from master.dbo.syslockinfo l, 

           master.dbo.spt_values v,

           master.dbo.spt_values x, 

           master.dbo.spt_values u, 

           master.dbo.sysdatabases d

      where   l.rsc_type = v.number 

      and v.type = 'LR' 

      and l.req_status = x.number 

      and x.type = 'LS' 

      and l.req_mode + 1 = u.number

      and u.type = 'L' 

      and l.rsc_dbid = d.dbid 

      and rsc_dbid = (select top 1 dbid from 

                      master..sysdatabases 

                      where name like 'my_db')

issue inside cursor:

SET @kill_process =  'KILL ' + @spid      

            EXEC master.dbo.sp_executesql @kill_process

                   PRINT 'killed spid : '+ @spid

after the cursor is closed and deallocated:

sp_dboption 'my_db', 'single user', 'TRUE'

go

sp_renamedb 'my_db', 'my_db_old'

go

DROP DATABASE MY_DB_OLD