Simple retrieves

Retrieve 1000,850,700 and 500 hPa temperatures from the 15th of February 1999 12 UTC Analysis > see in catalogue 

retrieve,
    class    = od,
    type     = an, 
    expver   = 1,
    date     = 19990215,
    time     = 12,
    param    = t, 
    levtype  = pressure level,
    levelist = 1000/850/700/500,
    target   = "t.grb"

http://apps.ecmwf.int/services/mars/catalogue/?class=e4&stream=moda&expver=1&type=an&levtype=sfc

Retrieve wind components for tomorrow at 12:00 and 18:00 from yesterday's 12Z forecast at all 91 model level

ERA 40

Retrieve the vertical integral of eastward and northward heat flux as monthly means of daily means on a 1.125 by 1.125 degree latitude/longitude grid over Europe from the ERA 40 archive for January 1964

retrieve,
    class   = e4,
    stream  = moda,
    type    = an,
    levtype = sfc,
    date    = 19640101,
    param   = 69.162/70.162,
    area    = europe,
    grid    = 1.125/1.125,
    target  = "e4_moda_fluxes.grb"