The words are fairly close in meaning, but here's how I keep them straight. If something is valuable, it's costly or worth a great deal, and we generally put a dollar figure with it. If it's invaluable, it means it's so important that I can't put a price on it.
1. Sue's engagement ring was valuable because it cost Ned $25,000.
2. Your expertise is invaluable—I could never have figured out how to set up my computer without your assistance.
In the second example, unless your friend charges you, the help is so deeply appreciated you can't put a money figure on what it's worth.
If it's valuable, I can put a price on it;
if there is no way to calculate the worth, it's invaluable.