teaching - How to teach addition of negative numbers?

There are some nice physical models of integers that I learned about when I was working with elementary school teachers. One that I find particularly useful is the following:

You have two kinds of chips (say, red and black, to correspond with the standard accounting practice). The rule is that a red chip cancels the black chip. If after that, if you have k black chips, that represents the positive integer k; if you have k red chips, that represents the negative integer -k, and if you got nothing, that's 0.

The first thing that a student needs to figure out is that there are a lot of ways to represent any integer, cause you can always add a pair of red and black chips. This eventually gets the point across that adding $(k-k)$ doesn't change the value.

Addition of two integers is easy: put both piles of chips together and figure out what integer you got.

Subtraction is not much more difficult: it's a take-away operation, analogous to what people are familiar with in counting numbers.
To do $a-b$ as a take-away operation, but you may need to modify $a$ by adding more pairs of chips to have enough to take away $b$ from it.

For example: 2-5
2 is represented five black and three red chips. Take away five black chips that represent 5, and you are left with three red chips which represent -3.

The other advantage of this model is that it's easy to demonstrate that $a-b=a+(-b)$. For example, consider 2-5 as 2+(-5): two black chips, put together with five red ones, which after cancellation yield three red chips representing -3.