SELECT * FROM SOMETABLE
ORDER BY 
CASE 
  WHEN monthfield LIKE 'jan%' THEN 1
  WHEN monthfield LIKE 'feb%' THEN 2
  WHEN monthfield LIKE 'mar%' THEN 3
  WHEN monthfield LIKE 'apr%' THEN 4
  WHEN monthfield LIKE 'may%' THEN 5
  WHEN monthfield LIKE 'jun%' THEN 6
  WHEN monthfield LIKE 'jul%' THEN 7
  WHEN monthfield LIKE 'aug%' THEN 8
  WHEN monthfield LIKE 'sep%' THEN 9
  WHEN monthfield LIKE 'oct%' THEN 10
  WHEN monthfield LIKE 'nov%' THEN 11
  WHEN monthfield LIKE 'dec%' THEN 12
  ELSE 13
END,monthfield

USE [tempdb]
GO

IF  EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[dbo].[testmonths]') AND type in (N'U'))
DROP TABLE [dbo].[testmonths]
GO

CREATE TABLE [dbo].[testmonths](
	[testMonth] [varchar](50) NULL
) ON [PRIMARY]

GO

INSERT INTO [dbo].[testmonths]([testMonth])
SELECT N'May-11' UNION ALL
SELECT N'May-12' UNION ALL
SELECT N'May-13' UNION ALL
SELECT N'Jun-12' UNION ALL
SELECT N'Jul-12' UNION ALL
SELECT N'Aug-13' UNION ALL
SELECT N'Jun-11' UNION ALL
SELECT N'Jul-11' UNION ALL
SELECT N'Aug-11' UNION ALL
SELECT N'Aug-12' UNION ALL
SELECT N'Jun-13' UNION ALL
SELECT N'Jul-13'

  
SELECT testMonth
  FROM testmonths
  ORDER BY CONVERT( datetime , '01-' + testMonth );