You must help a person in such a way that the said person doesn't even realize that they've been helped. Those are the best kind of helps.
Step-up to the plate and offer your shoulder when you feel someone is really in need of one but make sure to do it in a very casual, subtle and nonchalant way. Do not make a huge fuss of what you're doing for them.
Then help the through that phase as best as you can and gently nudge them back to happiness and laughter but ever so lightly that the other person doesn't even perceive of it.
(Of course, without ever losing sight of the fact that you can, at the most, be a catalyst. The real healing has to, and always does, come from within)
And - this is the most important step - once you're sure your work is done and they've overcome sufficiently, start to step back into the shadows so slowly and *imperceptibly* that, again, they don't notice you withdrawing into the crowd.
In simpler terms..
Know your role, Play your part & Leave the stage gracefully as soon as you get the hint that you're not required anymore.
Never overstay your welcome.
(And never EVER complicate what's simple!)