I came across this easy way to teach mental addition that I just had to blog about it! It is super easy to follow simply because it makes you start with the larger place value and flows from left to right (coincidentally it's also called the left to right addition). Here's how we do this with three digit numbers:
First, add the hundreds, then add the tens and last the ones.
Next add these sums together.
Here's a link to some fascinating videos that explain this mental strategy: