If I'm feeling uninspired I look for a fabric that has colors that appeal to me and use that as a starting point for my palette. I don't even necessarily buy or use the initial fabric, it's just for inspiration and ideas.
Learning a bit about color theory might help too; there are certain relationships that work well together on the color wheel and once you learn those it helps make it a lot easier to decide what "goes" together.
I mostly rely on my own intuition and taste, though. I lay some fabrics together and I can just tell if one isn't playing nice with the rest; it stands out and just doesn't look quite right. I think most people have this ability, they just don't trust their own intuition! Don't second-guess yourself too much. If you like it, you like it, go ahead and use it! And if you don't...try something else until it clicks!