I don't often get stomach aches, but when I do, I take wormwood. It also treats and prevents nausea, so I'll take wormwood if I'm anticipating nausea as well (like from certain supplements). It is absolutely foul tasting, so if you plan on taking this for when you get sick, you might want to familiarize yourself with the taste first. That said, there really is nothing better for digestive issues.
You can take it as either a tea or a tincture. You can take the tincture by putting a dropperful in a small glass of water. For the tea, I use one teaspoon in one cup of hot water and steep for five minutes. When I have a stomach ache, I won't bother bringing the water to boil since I want to drink it as soon as possible.