SP With Variable Number Filled Of Parameters

I have a SP search_post (e.g)

@id int,
@author varchar(40),
@keyword varchar(40),


select * from
posts
where
id = @id and author =@author and message like @keyword


in my case a user can choose to search by one or more of this elements.

what will be the best approach to write a statement that will account for the posibility of the user leaving some "parameters" empty, while also allowing for the posibility that all parameters could be filled


one approach I have thought out is this

if (@id IS NOT null)
select * from
posts where id = @id


if (@author IS NOT null)
select * from
posts where author= @author



if (@keyword IS NOT null)
select * from
posts where keyword = (@keyword



but this does not really take care of the posibility that all of them or some of them will be null while others will not

any suggestions ?