Consider the following if statement:

if(dragOperation != NSDragOperationCopy && NSPointInRect(currentMouseLocation, self.bounds)){
    //do something
}

Even though you may have worked out what the condition represents, it probably took you a little longer than it should. It's complicated, making it time consuming to read, and prone to bugs upon modification. Thankfully, there is an easy remedy:

BOOL isMovingWithinSelf = dragOperation != NSDragOperationCopy && NSPointInRect(currentMouseLocation, self.bounds);
if(isMovingWithinSelf){
    //do something
}

By using the variable name to explain the intent of the condition, the code is made more readable. This technique is also useful when the condition is simple, but the intent is unclear. Consider this example:

if([commandLineArgs containsObject:@"-scm"])
    //...

"-scm" could mean anything. It would be much clearer if it was written like so:

BOOL isInSuperCoolMode = [commandLineArgs containsObject:@"-scm"];
if(isInSuperCoolMode)
    //...

When written this way, it is obvious that the "-scm" command line argument activates "super cool mode."