apparently as mentioned above, proving the "opposite" has worked too...show that you are preparing to move.... get estimates from moving comanies... letters written to current employers to explain the upcoming departure, listings if you are selling property, letters to landlords explaining the intent to leave on PR receipt... if you can't prove that you are going, try to demonstrate that you are not staying
finally, when at a total loss, many have succeeded with a letter from family or close friends saying they will help you get on your feet and get settled once you land...
if you fail to provide them with any sort of proof they will come back and request it... not sure a written plan will suffice, but if that's all you have you can try...
good luck