If you are using Sql Azure then one option is to use Azure Federation to do exactly that.
In a very simplistic term if you have TenantId in your table which stores data from multiple tenants then before you execute a query like SELECT Col1 FROM Table1, you execute USE FEDERATION... statement to restrict the query results to a particular TenantId only, and you don't need to add WHERE TenantId=@TenantId to your query,
USE FEDERATION example: http://msdn.microsoft.com/en-us/library/windowsazure/hh597471.aspx
Note that use of Sql Azure Federation comes with lots of strings attached when it comes to Building a DB schema one of the best blog I have found about it is http://blogs.msdn.com/b/cbiyikoglu/archive/2011/04/16/schema-constraints-to-consider-with-federations-in-sql-azure.aspx.