This is NOT a situation we want. So I do lots of practical adding, with manipulatives, especially Base Ten/Dienes.
Starting right at the beginning, teach the concept of addition using an adding machine. This has been the best idea we have found - really cements into the children's mind that you are combining two numbers.
Click on the picture to find the original source.
Moving onto two digit numbers and units (TU+U). Hundred squares are your best friend.
I like to get the kids adding multiples of 10 and then tens and units numbers that do not go over the 10s boundaries. Loads of practise with Base 10, 10p and 1p coins, bundles of 10 straws and single straws. Anything to drill it into their minds that you can separate the 10s and 1s and add them more easily.
Finally, go over 10s barrier. I introduce this slowly, practise sums which do not go over the 10s and then throw one in that does. Through careful questioning such as:
I've never had a group of children yet who haven't worked out for themselves that we should swap 10 units for 1 ten and add it to the tens column. By solving the problem themselves, they will really understand why that pesky 1 (or more) gets carried under in column addition!
And then, repetition, repetition, repetition to hone the skills. Instead of endlessly repeating the same activity, boring the children AND myself, games will practise the skill and add a bit of fun.
Some of my favourite are pinned to my addition and subtraction Pinterest board which can be found by clicking the picture.
There's also this computer interactive which is useful to demonstrate methods:
There's also this computer interactive which is useful to demonstrate methods: