You could always buy a Shimano flight deck computer if you are using Shimano. It shows you the rings you are on in the display. For me it is pretty easy since I am almost always in my big ring and during normal riding I generally only shift my cassette down at most 3-4 gears. So I can mentally keep track and when I come to a stop I either do a double gear cassette change back + 1 = 3 or two double gear changes = 4. As CC says it is also pretty easy to look back and see what gear I am in. I never shift into my 25 or 11.

I wish Shimano go do a double shift down gears like it does up, that would be nice.