Stewing or slow simmering can tenderise certain types of cuts and bring out their best flavours (use flank, brisket or chuck – avoid lean cuts as they’ll get tough when cooked this way).
Over-cooking can make the meat dry and stringy. If the sauce hasn’t reduced but the meat is tender, remove the meat (and keep it covered so it doesn’t dry out) and heat the sauce at a higher temperature to reduce. Then return the meat to the hot sauce. You can also use cornflour to thicken the sauce.