I suppose it's what you're interested in, If you're after a very sensitive method with a very low limit of detection, even if it takes ages then I'd look at Liquid Liquid extraction. If you want high throuput then look at
Turboflow first, then SPE. If you want something that's quick to develop and gives acceptable results then try Protein Crash in organic solvent followed by centrifugation.
Check out some of these (and other) papers in the area;
Xu, R. N., Fan, L., Rieser, M. J., & El Shourbagy, T. A. 2007, "Recent advances in high-throughput quantitative bioanalysis by LC-MS/MS", Journal of Pharmaceutical and Biomedical Analysis, vol. 44, no. 2, pp. 342-355.
Zimmer, D., Pickard, V., Czembor, W., & Muller, C. 1999, "Comparison of turbulent-flow chromatography with automated solid-phase extraction in 96-well plates and liquid-liquid extraction used as plasma sample preparation techniques for liquid chromatography-tandem mass spectrometry", Journal of Chromatography, vol. A.. 854, no. 1-2, pp. 23-35.
Herman, J. L. 2002, "Generic method for on-line extraction of drug substances in the presence of biological matrices using turbulent flow chromatography", Rapid Communications in Mass Spectrometry, vol. 16, no. 5, pp. 421-426.
Just my two cents....
Paul.