Some things that I rarely conciously think about in daily work, but that can be insanely difficult to fix down the road:

1. Always set the environment for every object that you create:

     SET ARITHABORT ON
     SET ANSI_PADDING ON
     SET ANSI NULLS ON
     SET ANSI_WARNINGS ON
     SET CONCAT_NULL_YIELDS_NULL ON
     SET NUMERIC_ROUNDABORT OFF
     SET QUOTED_IDENTIFIER ON

2. Avoid outer joins unless absolutely necessary.
3. Avoid NULLable columns whenever possible.
4. Use ISNULL() all the time.
5. Use CASE WHEN myCol IS NULL THEN x ELSE y END all the time.
6. Never create NULLable keys.
7. Always create a human-readable 'Candidate Key' column along with the PK.
8. Work only from scripts (or regenerate the scripts after each change).
9. Examine the execution plans of all queries containing one or more joins.
10. Use SSMS and save scripts in a solution all the time.