adding to what has been said previously-
As a British/Irish gal living in East Anglia (it's complicated) I try to use words that are understandable by all and anything that might be a little outside the common ground of American TV/Films I try to explain in brackets.
Personally my favourite descriptive words for Bottom are :-
Backside - it does what it says on the tin
Behind - ditto
Rear - ditto
I don't like, just because I don't like how the words sound :-
Butt
Caboose
Bottom
Sit-a-pon (very old fashioned think prim little old ladies)
Ass
Arse
Buttocks
I must admit I don't really even like the word bum in my writing, and when I refer to someone getting their thighs slapped I mean the backs of them - like some Mother's used to do often it seemed in the 1970's. I must admit I know from experience how much the **************** sting!
Maybe I'm a prude, I don't know, but one thing I am going try as an experiment is trying out other words for Bottom as found in Roget's Thesaurus - I am sure there are other thesauruses(?) available I just know the name of this one so sue me!

Equally I find it amusing that American's call Bum Bags 'Fanny Packs' because fanny in British English means tbe front of a woman's private area. I suppose that might be why Fanny was used as a female name in some Carry On Films (PhilK - can you confirm?)
I'll shut up now.
pim