I have written a gag and got very mixed results with telling it.
(you might realise why after reading it)
So without anymore preamble, I throw it open.
The other night I had the weirdest dream.
I dreampt I was eating a giant marshmallow
and when I woke up
My flatmate had is cock in my mouth
Um... I get it. What's not to get? But I didn't really find it funny. Sorry.
Sorry, I didn't laugh. I can't really think of anything constructive to improve it either.
I liked it, a nice twist.
I like the twist from the classic pillow eating gag, btu I think it would be funnier if it had nothing to do with things in mouths at all, to completely wrong foot the audience:
"Last night I dreamt I was eating a giant marshmallow. But then I woke up and found...my flatmate was anally raping me whilst my Dad filmed it dressed as a bishop"
Or something 
Oh, I didn't get the whole 'we were supposed to think it was a pillow' thing.
Quote: zooo @ May 5 2012, 4:06 PM BSTOh, I didn't get the whole 'we were supposed to think it was a pillow' thing.
Me neither.
Nor me. I just thought they were... you know... good flatmates.
And there is the crux with this gag. I have seen the blank faces on people I have tested with it and inversely the big laugh.
I suppose a lot don't know or don't immediately associate this gag with the pillow gag.
I could never use it on an American audience - they really have never heard the pillow gag.
I agree with gappy in that it needs to completely subvert the "things in mouth" idea that the audience will be expecting.
Personally I'd go for something like:
The other night I had the weirdest dream.
I dreamt I was eating a giant marshmallow!
But when I woke up I found I'd actually
suffered a massive stroke.
Still not exactly side-splitting, but with a good delivery it could work.
Both versions seem to have an unpleasant sexual element for me.
I got the pillow bit but I think the blank faces were because the marshmallow is not instantly recognisable as a cock substitute is it??? 
To borrow your joke Stephen I'd have,
Last night I dreamt I was eating a giant marshmallow, when I woke up my flatmates big beer belly was now a sixpack, so I drank that too.
Quote: Shandonbelle @ May 6 2012, 10:47 AM BSTI got the pillow bit but I think the blank faces were because the marshmallow is not instantly recognisable as a cock substitute is it???
I think you have been away from Cork too long young lady!
Very true, don't tell anyone what I have become Marc 
Thank you all for your comments. I shall have to file in the 'not quite good enough' folder
And the original version ended with 'my father had his cock in my mouth'
I fancied that it was a Jimmy Carr'sh type joke.