Best ways to lower Student Loan interest rates?

I have a about 6 loans through sallie mae and about 5 or so through federal. What would be the best way to lower my interest rates, most of the sallie mae ones are like 6.8% and the federal ones are around 6.5%. I have been out of college for about 2 years and have been paying them back myself for about 1 year now so any way to lower the interest rate would be great. All information relevant is welcome.