The infamy will gradually wear off, so you have to take it VERY gradually. If you use a "Cassus Belli" ("cause of war") as an excuse (In HTTT and DW you can select any that are available when you declare war), the Infamy is sometimes discounted. Picking up a couple of provinces at 1/2 or 1/4 the Infamy is a big help. Gobbling up a 1-province minor country (OPM) without a C-B costs 8 Infamy, which you can sometimes get for 2 or 4 instead. Vassalizing a 2 or 3 province country, then annexing them diplomatically after 10 years, gives you a 4 point hit for the whole set, followed by another 4 points when you annex; the per-province cost is lower. In cases such as "holy war", etc., you can grab non-capital provinces for 1 point. Taking them from other HRE states for full Infamy cost is just painful.
Note that the more "illegal" provinces you own (where you don't have a "core" in that province), the less favorably the other HRE states will view you. You gain cores either after 50 years of ownership, or via events and missions. I often avoid confrontation with neighboring HRE states until I get a mission that grants me a core on a province of theirs for taking it. Playing as an HRE member, you usually need to expand very gradually, waiting until you shed Infamy and until you gain cores on any annexed land before taking more.