Slicing across the grain: Muscles are made up of many fibres in bundles running in the same direction. They give the meat texture and bite but can be tenderised by slow-cooking which breaks down the collagen holding them together, or by mincing or pummelling with a tenderising mallet. Another option is to slice across the muscle fibres to break the long strands leaving you with tender slices. Have a look for the fibres in the meat and slice horizontally across them.
If you can make your own stock you will get the best results but there is a cheat that works almost as well. Pick up a packet of beef flavour instant noodles from an asian supermarket and use the powder that comes with it to make a stock with boiling water. Then just add the noodles and fresh ingredients to turn it into something more special.