Subtracting Signed Numbers
Subtracting a negative number seems confusing. Even when you use a number line, do you go left on the number line because you're subtracting? Or do you go right because you go left because you're subtracting and then turn around again because the number is negative? It's confusing just talking about it.
Have you ever heard someone say "I don't want to take you to the amusement park because you're too young." And the response is "Please don't not take me to the amusement park?" Then you might hear "Don't use a double negative. Instead say "Please do take me to the amusement park instead." Two negatives makes a positive. Subtracting a negative number is the same thing. It's the same as adding a positive." Not so confusing, right?

Video Tutorials
Here's another example of subtracting a negative that describes "taking away a debt".

Here's an excellent explanation of adding and subtracting negative numbers using hot air balloons as an example. The only thing I didn't like is his description of an "infinite number of balloons and sand bags", really what he means is a balanced number of balloons and sand bags that makes the hot air balloon hover at 0 (but this is just a minor detail).

Other Tutorials
NRich  great example using a hot air balloon that explains adding and subtracting negative numbers.
Math is Fun  Adding and Subtracting Positive and Negative numbers. This site uses lots of different ways to explain the topic. It also has the Casey Runner game and online exercises.
Cool Math  The first two pages explain how to subtract signed numbers. The third page shows how to subtract negative numbers.
Online Activities
Practice Worksheets
Dad's Worksheets  Select the level of problem you'd like to practice under addition and subtraction of negative numbers. Answers are available to double check your work.
Kuta Adding and Subtracting Positive and Negative Numbers  Answers are available to double check your work.