  Evolution in the mass function of galaxy clusters sensitively traces both the
expansion history of the Universe and cosmological structure formation. Robust
cluster mass determinations are a key ingredient for a reliable measurement of
this evolution, especially at high redshift. Weak gravitational lensing is a
promising tool for, on average, unbiased mass estimates. This weak lensing
project aims at measuring reliable weak lensing masses for a complete X-ray
selected sample of 36 high redshift (0.35<z<0.9) clusters. The goal of this
paper is to demonstrate the robustness of the methodology against commonly
encountered problems, including pure instrumental effects, the presence of
bright (8--9 mag) stars close to the cluster centre, ground based measurements
of high-z (z~0.8) clusters, and the presence of massive unrelated structures
along the line-sight. We select a subsample of seven clusters observed with
MMT/Megacam. Instrumental effects are checked in detail by cross-comparison
with an archival CFHT/MegaCam observation. We derive mass estimates for seven
clusters by modelling the tangential shear with an NFW profile, in two cases
with multiple components to account for projected structures in the
line-of-sight. We firmly detect lensing signals from all seven clusters at more
than $3.5\sigma$ and determine their masses, ranging from $10^{14} M_{\odot}$
to $10^{15} M_{\odot}$, despite the presence of nearby bright stars. We
retrieve the lensing signal of more than one cluster in the CL 1701+6414 field,
while apparently observing CL 1701+6414 through a massive foreground filament.
We also find a multi-peaked shear signal in CL 1641+4001. Shear structures
measured in the MMT and CFHT images of CL 1701+6414 are highly correlated.
