Why is it that when you first fill up a balloon, it's hard to get air through, but after inflating it a bit, it becomes much easier to further inflate the balloon?
I think that most of the answers here are incorrect since it has nothing to do with decreasing resistance of rubber. In fact, the force required to stretch the balloon increases, not decreases while inflating. It's similar to stretching a string, ie. the reaction force is proportional to the increase in length of the string - this is why there is a point when you can no longer stretch a chest expander.

The real reason that initially it's hard to inflate the balloon is that in the beginning, ie. with the first blow, you increase the total surface of the balloon significantly, thus the force (pressure on the surface) increases also significantly. With each subsequent blow, the increase of the total surface is smaller and so is the increase of force. This is the result of two facts:


constant increase of volume with each blow
volume of the balloon is proportional to the cube of radius while surface of the balloon is proportional to the square of the radius


For a sphere you have:

$$
A={4}\pi R^2 \\
V={4\over3}\pi R^3
$$
The equations says that the amount of work required to increase the volume of the balloon by one unit is smaller if the balloon is already inflated.
