There is a lot of overlap in the Influence skills.
The way I adjudicate them is as follows:
Charm The touchy feely stuff.
Anything from seduction to playing on someone's
heart strings.
If your attempt to influence is based largely on
appealing to someone's emotions, I will probably
default to Charm.
Diplomacy I treat as an upscale version of Streetwise.
If you are dealing with nobility or the 'educated'
classes or trying to get something done through
a bureaucrat, whether legal or illegal, I will
probably default to Diplomacy
Duping Short term cons. Whether an actual con or trying
make the guard think you are just passing by,
if it's something the target will realize is a lie
once you are gone and they sit down and think about it
I will default to Duping.
Public Speaking If you are trying to Charm or Dupe a large number of
people at the same time, I will default to Public Speaking
Keep in mind that I will take the crowd's mood into
account when assigning bonuses to the roll.
Streetwise Not really an influence skill. More a knowledge skill
that I will use to provide general information about
all things shady.
Streetwise will let you know who you can bribe.
The actual bribery itself will depend on either another skill
or general roleplay.
I strongly encourage the use of roleplay when you call for any kind of influence check.
I will usually assign an bonus to your roll based on your post. Even if it's a bad attempt at roleplay, I'll award something just for the attempt.
If your post is good enough, I may just handwave the check.