I often find myself with an idea for a sketch but no punchline, so I start to type up the sketch and inevitably the punchline appears as i do so (some good... some often not to good).
Anyone else use this haphazard approach?
Punchlines
Yes, pretty much exactly.
My sketches tend to be about the situation, I leave it to my err "wit" to come up with the punchline when I'm writing the dialog. - That's why it's normally a stupid pun or something 
You don't need punchlines - Monthy Python proved that. But I would go along with what Leevil says. A lot of my sketches don’t have punchlines at first, but one suddenly comes to me while I'm writing it.
Yeah, that often happens to me as well. It just magically happens and when it does (slight pause) it's magic.
There's nothing stupid about puns.
Well you're going to have egg on your face, you must be yoking?
Its the voices in your head that tell you the jokes.
I find it is often a scene or an idea of some funny dialogue that gets me going but I think the best way is when you can start with the punchline and work backwards.
That way you can foreshadow the punchline, progressively build towards it and build the best scene possible to ensure greatest effect.
It's very rare I start anything until I see the end. Beginning something with no end in sight is too scary for me.
But when we're filming a written piece, we allow time to brainstorm new ideas as and when they suggest - and this has often come up with better punchlines than we started out with.
For once, this is the plus side of working with other people. 