Summarize:

An angled or sloped bottle helps to keep the milk at the top of the bottle, where the nipple is, even when your baby moves the bottle around. This keeps the nipple full at all times. These bottles are specially designed so that air cannot reach the milk. The nipple is separated from the bottle by a one-way valve. Use a bottle with a valve at the top. This valve allows just enough air to enter the bottle so that your baby can drink easily, without consuming any air bubbles. In this case, the straw is not actually used for drinking. Instead, the straw acts like a vent, so it makes it easier for your baby to drink out of the bottle without drinking any air bubbles.
Use angled bottles. Try a bottle with a one-way valve. Try a bottle that has a straw vent.