Context. 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. Aims: 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. Methods: 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. Results: We firmly detect lensing signals from all seven clusters at more than 3.5? and determine their masses, ranging from 1014 M&sun; to 1015 M&sun;, 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. Conclusions: We confirm the capability of MMT/MegaCam to infer weak lensing masses from high-z clusters, demonstrated by the high level of consistency between MMT and CFHT results for CL 1701 6414. This shows that, when a sophisticated analysis is applied, instrumental effects are well under control. Observations reported here were obtained at the MMT Observatory, a joint facility of the Smithsonian Institution and the University of Arizona.Appendices are available in electronic form at http://www.aanda.orgReduced and coadded MMT image files are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A A/546/A79
