My answer depends greatly on the tires used and whether the LSD is set more open or more locked. With a more open LSD and Sports tires, you want to keep the ratios short (higher top speed) so as to not break loose readily, while Racing tires and a more closed differential affords you the opportunity to tune the transmission tall for increased acceleration. Beyond that, do a little free roaming on the track and figure out where the engine is peaky (it's a bike engine, it's peaky) and use that to maximize the amount of torque on tap immediately after shifting. If you don't plan to use that particular car (the one you're tuning for ARM, not the model of car) on other courses, tune the box to minimize the shifting between corners and set it up so that you shift immediately after apexing and the natural reduced speed minimizes time lost. This, however, won't be such a good idea with the aforementioned lower grade tires and/or open differential as shifting and acceleration immediately after apex will induce wheelspin. If you have problems in this area after tuning the box, shift prior to apex in order to take advantage of a lull in the power curve.
I tend to keep my tunes to myself, not for the sake of advantage but because everyone drives differently and what works for one person won't always work for another, but these basic guidelines should help you improve your times considerably.