We are currently using Math-U-See and Gess seems to do pretty well with it, though we are moving through it rather slowly. While studying addition they want you to memorize your math facts. We did alright with that for the zeros and ones, but Gess got stuck trying to memorize beyond that, so we went ahead and moved on. When we got to the doubles I decided to pull out a tool from the book Teaching Math to People with Down Syndrome.
For teaching doubles they have some visual cues that you can copy and make cards out of. Now they don't have pictures for 7 and 8 so I made my own. They suggested using crayons for 8 so I got me a picture to do that with and I could not think of anything for 7 so we just made a domino with 7 on it. It seemed to work. We pasted the pictures on index cards. One side shows the answer and the other side does not.
Now, some of these images assumes that the person already knows that number is a double. Gess however, did not know a bug had 6 legs (3 on each side) and a spider had 8, etc. So each day we learned one double fact, starting with 2. We were not only learning the double, but the image concept to help her memorize it. The eggs were the funnest, because when we got to that one we went to the fridge and counted a dozen eggs with six in each row (the concept of a dozen was new to her too). She really liked that.
Now that we knew what the pictures represented and why we started drilling with the cards. First she would just use the sides with the answers to learn them and then I started drilling with the side without the answers.
Then we added the use of the cards along side her math workbook.
And then finally she had them memorized and was able to answer the questions without the use of the cards.
You should note that we have to drill memorized math facts regularly or she will lose them. I usually drill her before each workbook page, especially if it has review on it. She gets the facts right almost every time when we use flash cards, but if the problem is in the middle of a bunch of different type of problems she seems to forget that she knows it and doesn't recall it well. Doing flash cards right before the workbook or test helps her to remember that she actually "knows" the answer.