Ultra-Compact PCB Downsizing

My current board is already under 4 sq inches, yet I still need to reclaim every possible millimetre of space. The single overriding goal is size reduction; cost and raw performance can remain as they are as long as nothing regresses... (Budget: $15 - $25 USD, Jobs: Circuit Design, Electrical Engineering, Electronic Design, Electronics, Microcontroller, PCB Layout)