Before the canning starts, you can help to ensure your success by picking out the right kind of tomatoes. You want to can at the peak of ripeness, so look for healthy, disease-free tomatoes. You'll also get a better result if your tomatoes are uniformly colored. Check out the skin to make sure it's tight and that there's no bruising. It's also important that the tomatoes you're using to can haven't been subject to frost while on the vine. This can knock the acidity down to unsafe levels for canning.