Guidelines for using properties vs methods

I often have a hard time deciding if certain data should be exposed through a property or a method. You can say "use properties for object state", but that's not very satisfying. Take this example for instance:

- (NSString *)stringOne
{
    return _stringOne;
}

- (NSString *)stringTwo
{
    return _stringTwo;
}

- (NSString *)mainString
{
    return [_stringOne length] > 0 ? _stringOne : _stringTwo;
}

It's clear that stringOne and stringTwo should be properties because they are clearly object state. It's not clear, however, if mainString should be a property. To the end user mainString acts like state. To your object, mainString is not state.

This example is contrived but hopefully you get the idea. Yes, properties are nothing more than a convenient way to create getters and setters but they also communicate something to the user. Does anyone have decent guidelines for deciding when to use a property vs a method.