I've got a slow cooker but for a roast I use the oven. My mother taught me this way of cooking a roast and it's turned out really tender and flavorful. I use a piece of silverside. Brown it in a skillet with your veg (chopped onions, carrots, celery, garlic, etc.). I throw in a little bit of sugar too, and some worcestershire sauce. When the meat is browned, put it and your veg into a casserole pot, then make up some chicken or beef stock (I use those cubes) and pour in enough to come up just over halfway up the piece of meat. Lay a piece of foil over the pot, then put the lid on (this really helps to keep the moisture in). Then roast in a 150 C oven for 3-4 hours. Once it's done, I take the meat and veg out and pour the liquid into a pan to make a gravy, then put it all back together. You can add some wine to the liquid too.
I suppose this could translate to a slow cooker too. I think browning the meat is an important step as it adds a nice flavor.