Whitespace

int i;main(){for(;i["]<i;++i){--i;}"];read('-'-'-',i+++"hell\
o, world!\n",'/'/'/'));}read(j,i,p){write(j/p+p,i---j,i/i);}
- Dishonorable mention, Obfuscated C Code Contest, 1984.
Author requested anonymity.

Use vertical and horizontal whitespace generously. Indentation and spacing should reflect the block structure of the code; e.g., there should be at least 2 blank lines between the end of one function and the comments for the next.

A long string of conditional operators should be split onto separate lines.

if (foo->next==NULL && totalcount<needed && needed<=MAX_ALLOT
	&& server_active(current_input)) { ...

Might be better as

if (foo->next == NULL
	&& totalcount < needed && needed <= MAX_ALLOT
	&& server_active(current_input))
{
	...

Similarly, elaborate for loops should be split onto different lines.

for (curr = *listp, trail = listp;
	curr != NULL;
	trail = &(curr->next), curr = curr->next )
{
	...

Other complex expressions, particularly those using the ternary ?: operator, are best split on to several lines, too.

c = (a == b)
	? d + f(a)
	: f(b) - d;

Keywords that are followed by expressions in parentheses should be separated from the left parenthesis by a blank. (The sizeof operator is an exception.) Blanks should also appear after commas in argument lists to help separate the arguments visually. On the other hand, macro definitions with arguments must not have a blank between the name and the left parenthesis, otherwise the C preprocessor will not recognize the argument list.