A variety of chemical modifications are being used to improve stability and potency of siRNAs for use in RNA interference. While no single cocktail of modifications is widely accepted as "best", LNA, 2´-O-methyl RNA, and other RNA modifications are commonly used today in "second generation" siRNA designs.
Braasch, D.A., Jensen, S., Liu, Y., Kaur, K., Arar, K., White, M.A., and Corey, D.R. RNA interference in mammalian cells by chemically-modified RNA. Biochemistry, 42:7967-7975 (2003).