Melbro,
please dont let this put you off the job. i know it can be really uncomfortable sometimes doing the demo when you feel people are sniggering and possibly talking about you. you can feel incredibly vulnerable standing up there with all eyes on you. when ive felt uncomfortable about the way someone is behaving, i tend to just stare straight at the person and try and make them uncomfortable back- you know, purposely make eye contact with them, just a blank faced stare, nothing nasty- and force them to break the stare and look away first- but thats not always the best way to deal with it either.
i dont think theres really an easy answer to it other than to try and just reassure yourself that they are just childish losers. and really thats all they are. for grown adults to think its amusing to make someone else feel so uncomfortable, particularly when your doing your job is just kinda pathetic.
also, its really very possible that they arent actually laughing at you. it maybe just appears that way. you know, maybe something you did, or the PA reminded them of something else that was funny?? its a possibility. ive been in situations before where someone was serving me in a restaurant and myself and my partner started killing ourselves laughing just before the waitress approached us, and i was soooo worried that she would think we were laughing at her- im really concious of things like that- but lots of people arent.
also, i guess if they had been making enough of a fuss that it was really distracting to you and the other pax around them, it might not have been inappropriate to ask them to keep it down a bit so that 'other passengers can hear the safety demo' - this is a valid issue, and ive actually had a passenger tell a couple of other noisy pax to keep it down for this very reason.
its tough at times, but sometimes you just have to be thick skinned- and maybe try not to assume the worst just incase they really werent laughing at you- easier said than done i know, i try and remind myself of this all the time!
and as i said above, please dont let it put you off your job or get you down. people like that just arent worth it.