  We describe a simple but efficient method for deriving a consistent set of
monopole and dipole corrections for multi-frequency sky map data sets, allowing
robust parametric component separation with the same data set. The
computational core of this method is linear regression between pairs of
frequency maps, often called "T-T plots". Individual contributions from
monopole and dipole terms are determined by performing the regression locally
in patches on the sky, while the degeneracy between different frequencies is
lifted when ever the dominant foreground component exhibits a significant
spatial spectral index variation. Based on this method, we present two
different, but each internally consistent, sets of monopole and dipole
coefficients for the 9-year WMAP, Planck 2013, SFD 100 um, Haslam 408 MHz and
Reich & Reich 1420 MHz maps. The two sets have been derived with different
analysis assumptions and data selection, and provides an estimate of residual
systematic uncertainties. In general, our values are in good agreement with
previously published results. Among the most notable results are a relative
dipole between the WMAP and Planck experiments of 10-15 uK (depending on
frequency), an estimate of the 408 MHz map monopole of 8.9 +- 1.3 K, and a
non-zero dipole in the 1420 MHz map of $0.15 +- 0.03 K pointing towards
Galactic coordinates (l,b) = (308,-36) +- 14 degrees. These values represent
the sum of any instrumental and data processing offsets, as well as any
Galactic or extra-Galactic component that is spectrally uniform over the full
sky.
