  Accurate calibration of data is essential for the current generation of CMB
experiments. Using data from the Very Small Array (VSA), we describe procedures
which will lead to an accuracy of 1 percent or better for experiments such as
the VSA and CBI. Particular attention is paid to the stability of the receiver
systems, the quality of the site and frequent observations of reference
sources. At 30 GHz the careful correction for atmospheric emission and
absorption is shown to be essential for achieving 1 percent precision. The
sources for which a 1 percent relative flux density calibration was achieved
included Cas A, Cyg A, Tau A and NGC7027 and the planets Venus, Jupiter and
Saturn. A flux density, or brightness temperature in the case of the planets,
was derived at 33 GHz relative to Jupiter which was adopted as the fundamental
calibrator. A spectral index at ~30 GHz is given for each. Cas A,Tau A, NGC7027
and Venus were examined for variability. Cas A was found to be decreasing at
$0.394 \pm 0.019$ percent per year over the period March 2001 to August 2004.
In the same period Tau A was decreasing at $0.22\pm 0.07$ percent per year. A
survey of the published data showed that the planetary nebula NGC7027 decreased
at $0.16\pm 0.04$ percent per year over the period 1967 to 2003. Venus showed
an insignificant ($1.5 \pm 1.3$ percent) variation with Venusian illumination.
The integrated polarization of Tau A at 33 GHz was found to be $7.8\pm 0.6$
percent at pa $ = 148^\circ \pm 3^\circ$.}
