Having removed lots of these over the years a couple more tips.
1. Can of PB blaster a day ahead of time if possible.
2. Tighten it a bit before you try to loosen it. You would be surprised on most bolts and screws how well this works not a bunch just give it a nudge to loosen the crud in the thread paths. The other tips listed here work also as often you will find them in rough shape to begin with.
