As always, not all of these will apply in all cases, and some are arguable (I disagree with their take on nulls and identities), but the rules and guidelines for SQL Server listed here and produced by SSQ are a good source to review:
http://www.ssw.com.au/SSW/Standards/Rules/RulestoBetterSQLServerdatabases.aspx
http://www.nyx.net/~bwunder/dbChangeControl/standard.htm