Have you started buying "good" mince? You need a certain fat content in the meat to get a moist loaf (unless you add things like soup or mayo.) For meatloaf I think the cheaper mince makes a much nicer loaf than the more expensive minced steak.
I'll tell you what I do, including the onion, which you can obviously omit if you wish. I start with using a fattier mince than I would normally use for other things, as PR suggested. But I set that aside for now and mix the other stuff first.
I finely dice onion, green pepper, and red pepper. (I chop the veg really fine so it isn't crunchy when the loaf comes out of the oven.) I crush up some crackers - usually just cream crackers -and mix them with 2 beaten eggs, making sure the crackers are softened and the whole thing becomes a bit gooey. Then I add about a half cup of ketchup to the egg mixture, along with the seasonings I am going to use - mostly I just use a bit of garlic granules, dried thyme, and salt and pepper.
After I have mixed all of that other stuff together, I then pour it over the meat and just work it in, without overmixing it. I plop it into the loaf pan and into the oven at 180C for about 90 minutes. Halfway through, I pull it out and spread a bit of ketchup over the top.
I mix all the ingredients separately so that the meat only gets mixed once. I've heard that overmixing it causes it to be dry, so I figured rather than add all the ingredients sort of separately and mix each one in, that it was better to mix all the ingredients in and then just mix it once. (Also, mixing the crackers with the beaten egg first means that there aren't any dry curnchy bits in the loaf.) I do pull the meat loaf out of the oven now and again and pour some of the fat out from the pan, to keep it from being too greasy, as a fattier mince will produce a lot more grease and the loaf would be soaking in all that grease otherwise, since I put mine in a loaf pan. Some people just form it into a loaf and put it in a roasting tray, which I guess would help, but I've never tried doing that.
Hope that helps!