What are some practical techniques/methods which can be used to train/improve your working memory and what studies have been done on this subject?
There are a few commercial packages that provide training for verbal and visuo-spaital working memory. The most well-known is CogMed (http://www.cogmed.com). Another popular one is Jungle Memory (http://junglememory.com). Both of these packages have been used in research studies (comment about these results below) There is also a myriad of online working memory games that promise to improve working memory, e.g. some games in Luminosity (http://www.lumosity.com). The content of these games is very similar, but there are fewer independent scientific studies of them as far as I'm aware.
As a caveat, the scientific evidence about the benefit of WM training is mixed at best. Training on WM tasks improves performance on them. This improvement also transfers to similar tasks (near transfer). However, there is currently little evidence for transfer to other tasks or improvements in everyday function (far transfer). For example, have a look at this report from a randomised control trial of working memory training in children: http://onlinelibrary.wiley.com/doi/10.1111/desc.12068/full
In recent news, Lumosity has paid a $2 million fine to the FTC for unsubstantiated claims about the benefits of its training. I've heard CogMed recommended by practicing neuro rehab professionals. Apparently, there is science to back its claims. It gets a bit pricey, however.