Fixity declarations are your friends
As we know often it is often cheaper to define an operator instead of a function (see this tip), sometimes it pays off to declare a fixity different from the default (
infixl 9). Imagine for example the following which is fairly common (57 bytes):
(a:b)!(c:d)=undefined (a:b)!e=undefined e!(a:b)=undefined
In that particular case using a fixity declaration is still 1 byte more expensive (58 bytes):
infix 4! a:b!c:d=undefined a:b!e=undefined e!a:b=undefined
As you can see you can use the low fixity to omit several parentheses, if somewhere in your code you have other parentheses that are due to the high default fixity, you can save bytes for these by declaring another fixity.
Note: This might seem like an arbitrary example but it is realistic as you can see in this answer.