It depends which side I'm playing. As the Union I rarely use the plunder option, but I can make Florida a Confederate headache by landing sailors in several cities. The Confederacy never has enough money for roads or trails, but a partisan ambush/raid, sea mine or submarine can make a big difference and they cost little to nothing.
Outposts can be a useful option for either side. Plant an outpost in a region bordering a river, place some artillery inside and it will bombard automatically via inside-fort rule. Much less expensive in time/money than a redoubt; they're only a few VP and one turn with a combat unit in the region. They can be put in relatively inaccessible areas but still be supplied. As the Confederacy, I usually plant them on the south bank of the Arkansas River, in every region bordering Fort Smith and Little Rock, put some militia and artillery in each. Athena has managed river travel along that river by bypassing the Mississippi river forts and using the option at Jacksonport, AR. Doesn't work for her now, and didn't cost me much.
Building up loyalty with the regional decisions, in border states and as you advance, can keep lower the probability the enemy will be able to raise partisans and Unionists/Copperheads to threaten your supply lines. For the Confederacy, demonstrations tend to be expensive but might tip the scales in a region that only needs a few points to create partisans or Copperheads.