I think the most important part of the charm person spell from this point of view is:
The spell does not enable you to control the charmed person as if it were an automaton, but it perceives your words and actions in the most favorable way
In other words the target may know you cast a spell, but so what. You are their best friend, their best friend casts spells. That's cool. Generally they wouldn't even question it and if someone else did question it they would jump to your defense as they perceive it favorably.
I.e. in a hypothetical situation:
A group of toughs in a bar, the leader is squaring up to the players, looking to start a fight.
The wizards casts charm person on the leader, who fails his save:
Wizard: Hey, no need for trouble. You know me remember, we met last year. I helped you out.
Leader: Oh wow, yeah. How are you doing mate?
Minion: But boss, he just cast a spell.
Leader: Of course he cast a spell, he's a powerful wizard! I bet he was just checking it's really me before revealing himself. We've had those assassins running around in disguise recently after all!
Wizard: That's right, can't be too careful. Now why don't you and your boys sit down and we can talk some business.
The thing is the charm person spell will actually make the target want to put the most favorable light possible on the fact you just cast a spell.
Clearly you do still need to think about. You need to pick the right target, in the right situation. Ideally you want to use the 30' range to cast the charm person before you even go in and strike up the conversation.
For example to get past guards at a gatehouse - stop 30' away, have your friends step between you and the guard to hide your hands while you can still see the target. Cast Charm Person, then walk up and start the conversation once he's already charmed. Depending on how alert the guards are they may get a perception roll to notice the spell casting, but a lot of the time they wouldn't even get that unless they made the save.
All of these strategies have both play and counter-play available to them, but that's part of what makes them interesting. Perhaps the bandit leader could instruct his men to ask him the name of any "sudden friends"... but how many would actually take that sort of precaution, and even then you are their best friend and they know they gave that instruction. They would make up an answer to cover for their "friend" .
"Uh, he's the Wizard Berlung, we hung out last year, or at least that's what he was calling himself then..."
This is harder, clearly if you cast a spell in front of someone they are going to suspect it. But this is a subtle spell, designed to be used in subtle ways. You have a 30' range on it, that's actually a fair distance. Wait for the target to walk past, cast it at them through a window, cast it in a noisy crowded bar.
The point is that this is an intrigue spell, one used for plots and cunning. No it won't work if you walk up to someone and cast it in their face, but neither should it. If you want to be cunning then be cunning.
For example, injured target on the floor (maybe the party rogue dropped a brick on their head from the roof). The witch runs over:
I'm a healer let me help, I know healing magic
The witch casts sow thoughts followed by cure light wounds one after the other.
Any magically knowledgeable onlookers might notice that the spell took longer than usual to cast, but no-one else would be any wiser, particularly when they saw the healing start to work.
You are pulling a con on a shopkeeper, the party face goes up and engages them in conversation, in the meantime the caster stays back and casts the spell looking in through the window. Or if acting solo then wait for another customer to distract the shopkeeper then cast the spell followed by going in yourself.
The point is these spells have weaknesses, but that just means you need to think about how you use them. If you want point-and-click then charm spells are not the school for you :)