Two factions are at civil war with each other, both offer "Massacre missions (kill X ships of the other faction) for sometimes pretty high credit values (10 Mio.+ if you're allied with them). One of the factions (New 17 Draconis Alliance) also has a small surface outpost where skimmers spawn if you approach, but lacking other defenses. Skimmers count as killed ships and each killed ship counts for all of your running massacre missions.
Get there with a quick ship loaded with rockets. Stack the missions of one of the factions (Jet Bridge Inc. iirc) to hell and back, drive to Faraday keep, shoot rockets into the skimmers, killing them quickly and instantly. Log out, log back in, kill a new set of skimmers, repeat until all missions are completed, drive back to the space station. get your 50-100 Mio. credits per run. Repeat. Only caveat: The outpost is guarded by a combat ship of the holding faction. Which btw. contributes to the effectiveness of the scheme, because it hardly is able to destroy you within the 15 seconds it takes to logout. Once you've completed all missions though, just let yourself be killed by the ship (or self destruct) and you're instantly back at the starport to take on a new set of missions.
Edit: In theory, this should also be possible in other systems. Does anybody happen to know an Empire system with a similar setup? The thing with this slightly exploitative method is, for military progression and credit gain, it renders every other activity the game offers completely moot. There's hardly any way to complete so many missions for a faction in such short time, which contributes greatly towards military progression. In terms of credits, it's just insane.
