Choosing function parameter types - pointers or objects?

At the moment, I only write functions which accept pointers as
arguments in order to change a value, but I've realised it would often
be a lot more effiecient to pass a pointer rather than an actual
object.
I know this question is a bit vague, but I was wondering about any
rules / guidelines / style issues, for when to use pointers, and when
not to, in your function parameters?

I'm also interested in any grey areas, or even off-topic functions that
have been written like in the win32 API for instance, but I doubt
anyone else is.

Cheers, Matt

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