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I've enforced a rule that combines a couple of what I've seen here.
You can only aid another if you're trained in the skill. And it's a flat 10 or better on a d20 roll - I'll give them a bonus if they come up with a good way they're aiding (something creative can get up to a +5 meaning they only need a 5 or better on the d20 roll).
People are much more intentional about skills this way.