Remember to let your "asker" down gently, it takes guts to ask in the first place and to be blown out for stepping up is pretty crushing. Try to make your response credible and a little bit believable, certainly don't be dismissive or haughty unless it's warranted, whatever hairy arsed beast rocks up to try their luck. And lastly remember if someone's into you they don't really want to end up in the friend zone.
In each case make your apology and then give your reason, e.g. I'm sorry but ...
Declining a drink
I'm 11 months pregnant
Thanks but I suffer from (excessive) water retention so have to be careful
Thank you but my ex / stalker's over there and he'll go mad
Shhh ... I've just stolen this handbag / purse
I'm on surveillance (I'm a copper / detective / spy)
Declining a Dance
I've got terrible bunions / corns / in-growing toenails
I sweat so badly, (option: I have over active sweat glands), that if I dance I'll be wet through in a couple of minutes
I said I wouldn't dance publicly again after I reached 50
Last time I danced somebody got really hurt
I have chronic Chorophobia (the fear of dancing)
You know I have 67 pairs of shoes and I bought the wrong ones
Declining Dinner
I'm too embarrassed, growing up in a slightly earthy commune I never got round to using a knife and fork.
Declining A Date
You're just like my last boyfriend and that ended so badly (I'd never want that to happen again, can we just be friends)
You're just like my brother so it would be too weird