To cite package flexmix in publications use:

Friedrich Leisch. FlexMix: A general framework for finite mixture models and latent class regression in R. Journal of Statistical Software, 11(8), 1-18, 2004. URL http://www.jstatsoft.org/v11/i08/

Bettina Gruen and Friedrich Leisch. Fitting finite mixtures of generalized linear regressions in R. Computational Statistics & Data Analysis, 51(11), 5247-5252, 2007. doi:10.1016/j.csda.2006.08.014

Bettina Gruen and Friedrich Leisch. FlexMix Version 2: Finite mixtures with concomitant variables and varying and constant parameters Journal of Statistical Software, 28(4), 1-35, 2008. URL http://www.jstatsoft.org/v28/i04/

Corresponding BibTeX entries:

  @Article{,
    title = {{FlexMix}: A General Framework for Finite Mixture Models
      and Latent Class Regression in {R}},
    author = {Friedrich Leisch},
    journal = {Journal of Statistical Software},
    year = {2004},
    volume = {11},
    number = {8},
    pages = {1--18},
    url = {http://www.jstatsoft.org/v11/i08/},
  }
  @Article{,
    title = {Fitting Finite Mixtures of Generalized Linear Regressions
      in {R}},
    author = {Bettina Gr\"un and Friedrich Leisch},
    journal = {Computational Statistics \& Data Analysis},
    year = {2007},
    volume = {51},
    number = {11},
    pages = {5247--5252},
    doi = {10.1016/j.csda.2006.08.014},
  }
  @Article{,
    title = {{FlexMix} Version 2: Finite Mixtures with Concomitant
      Variables and Varying and Constant Parameters},
    author = {Bettina Gr\"un and Friedrich Leisch},
    journal = {Journal of Statistical Software},
    year = {2008},
    volume = {28},
    number = {4},
    pages = {1--35},
    url = {http://www.jstatsoft.org/v28/i04/},
  }