An investigation of the mechanisms that can play a role in limiting the electrical connectivity in polycrystalline MgB2 is reported. Bulk samples have been manufactured by three in situ techniques using different types of precursors. The “internal magnesium diffusion” route has been studied in detail, in particular for what concerns the effect of the excess of Mg in the reagents on the electrical and structural properties of the samples. A modified internal magnesium diffusion method, which involves the use of MgB4 in the place of B, is also proposed. Electrical connectivity higher than 70% has been achieved also in the case of samples not fully densified. Our study indicates that both the excess of Mg and the use of MgB4 in the place of B can allow limiting the effect of insulating phases on the electrical transport, thus increasing the connectivity. The obtained results appear very promising to synthesize highly dense and electrically connected polycrystalline MgB2.