What are some good apps or training that improve working memory?

Imimic: First make sure your nutrition (including B vitamins) and sleep are excellent, and watch drugs/alcohol. Then remember you learn best by learning something, giving it time, and coming back to it. The longer the time the better. iMimic is very good for rapid memory. But you can even practice with a simple sudoko board. Memorize the numbers, give it time, and come back.