Just a note that in the UK when people say 'pants' they mean 'underpants'. We use 'trousers' for when Americans use 'pants'.

In the US, we most often say "pants."  Occasionally someone will use the word "trousers."  Everyone understands the word "britches," but as someone said, the word sounds a bit funny. 

However, there are a few idioms which use the word "britches."  Take a look:    http://idioms.thefreedictionary.com/britches

I usually use pants, but sometimes i use britches too. Depends on which kind of cloth i use with it.

Pants is much more common, but I use britches a lot because it's funny :-)

Others include jeans, trousers, slacks, khakis