Get the Whitepaper called Indexed Blind SQL Injection. Time based blind SQL attacks suffer from low bit/request ratios. Each request produces only one valuable bit of information. This paper describes a tweak that produces higher yield at the expense of a longer runtime. Along the way, some issues and notes of applicability are also discussed.
You can download it from the following link: https://packetstormsecurity.com/files/download/107506/indexed-blind.txt
Source: https://packetstormsecurity.com/files/107506/Indexed-Blind-SQL-Injection.html