I have tried two ways yet:
Starving the garrisons by excessive use of logistic strikes, this proved very successfull but led to problems with my own advance.
Direct penetration of the maginot line at the outbreak of war proved most effective. For this purpose, I created three divisions specially designed for storming fortresses (in my latest game they consisted of 1 inf/ 2 eng/ 1 art) and attached it to an army which had exclusively fortress buster guys in its ranks. In the first week, when the french hadn´t fully mobilized yet, this force virtually sliced to the maginots like a hot knife through butter. The initial attack was aimed at its northern wing, where the storm troops wouldn´t have to attack across the Rhine. Without difficulties they made a fine breach into the French main defensive line (which now was a very comfortable German main defensive line) and then moved towards Metz, mobbing up the two remaining fortresses there, while one Panzer Army arriving from Poland exploited the gap. Another Panzer Army advanced via Luxembourg/ Belgium, for this purpose it was very comforting to have the Metz stronghold annihilated so you don´t have to waste troops to lay siege on it. Having dealt with the northern maginots, I sent the stormtroopers for its remnants in the south, which are usually a harder nut to crack because of the Rhine protecting it from the west and French reinforcements having arrived in the meantime. On the other hand, the French troops trapped here wouldn´t stand in the way of my armored spearheads heading towards Paris and other vp provinces. It is a matter of taste mopping up the southern fortresses as well or just sit there until the vichy event fires. Due to the intensive and long fightings my stormtroopers now are the most elite(40+ experience) formations of the entire Wehrmacht.