All the other responders had great advice so no point in me saying the same thing. However, I will say this...the #1 trick to having a good tasting crockpot item is: Brown the item in a skillet before you proceed with the chosen recipe. Maybe not every item needs to be browned but I promise you it makes a world of difference in the flavor.
Example: If you are making a beef stew, take the cubes of meat, dust them with seasoned flour and then just sear them in a skillet with a bit of oil. Then, put the meat into the crockpot and proceed with cooking. Root vegetables like: carrots, onions, garlic, parsnips, beets & turnips also taste great when they've been browned. You will also enhance the color/appearance of your dish. The biggest complaint most people have is that "when you cook something in a crockpot, everything starts tasting the same". That isn't true once you brown or sear food items.
One of my favorite recipes is a Guiness Beef Stew. Just take your basic beef stew recipe, brown all the food items (beef cubes, carrots, onions), add in beef STOCK (not broth), seasoning to taste (sea salt/fresh cracked pepper) and then 1 hr. before serving, add in 1 cup of dark Guiness Beer. Just before serving, stir in 2 T. of butter. I had this dish in London on a cold winter's night. It was fabulous.
I'm a personal chef.