I love playing the HRE mainly because of location. In my experience an early blitz is the best strategy.

Army 1, Heinrich the Champion takes Florence, ask for a crusade on Tunis. On the way there I take Ajaccio and Cagliari. When Tunis is secured I give it to either the pope or the Sicilians. Now, Heinrich´s stats are almost maxed out wich will help you in all future negotiations.
Army 2, from Vienna and Huremburg takes Prague, Breslau and Stettin.
Army 3, from Frankfurt takes Hamburg and Magdeburg.
Army 4, from Staufen and Innsbruck and Metz.
All finished by turn 5. Army 2&3 merge and march on Antwerp and Bruges which should be yours around 8 or 9.

I only keep castles in Innsbruck and Magdeburg. I ally with all my neighbours except Milan (and Venice sometimes). My alliances always contains a clause that they pay me 100f/ turn for 20-50 turns. This will keep the peace in most of my game for a long time. My main focus are Italy and the rich cities of Genoa, Milan and Venice. Usually Milan goes for Venice and eventually gets x:ed. This is what I´ve waited for.
I always calls the crusades and sends my faction leader, heir and sometimes others to keep their stats maxed and my rep as high as possible. This soon gets my rep up to very trusworthy and hardly none touches me from that point.