When selecting a water bottle to use in the experiment, it is important to think about bottle size and shape. A larger bottle with a wider opening can hold more, but may not produce as much lift as a smaller bottle with a narrow opening. In a bottle with a smaller opening, the gas produced from the acid-alkaline reaction accelerates when squeezed through that smaller opening, creating more lift.
Not all acids are created equally. Some substances are more acidic than others. For example, vinegar is known to react with the alkaline baking soda; they are the key ingredients in the homemade volcano science project. However, those ingredients create a slow, steady reaction -- not a strong, fast one needed to produce lift. You may get a better reaction by using a highly acidic substance, such as concentrated lemon juice.
In order to make sure the water bottle rocket does not react at the wrong time, it is important to wrap the alkaline baking soda in a packet that can be inserted at the last minute. Using a thin paper that will dissolve slowly allows time to prepare the rocket before it launches. Coffee filters, wrapping tissue or paper towels are useful wraps.