Simple : Port should always be stored horizontally (lying down).

To keep it alive Wine of any form needs a 'seal' from the outside world, which is achieved by the cork. When moist the cork swells in the neck and forms a seal, and the cork is kept moist by being in contact with the wine.

Unfortunately it is not quite that simple :
- over the years the cork loses it's elasticity, so although it may have always been in contact with the wine it will eventually fail and leak, and bottles will therefore need to be Re-Corked about every 40-60 years. A good wine merchant can help you with this.

- secondly, the wine slowly evaporates through the cork, (causing ullage).

If you intend to cellar wine for any length of time then you need to control the environment in which it is stored. Preferably store at 12°C and high humidity. However the stability of temperature is more important than the exact temperature, so install a themometer and record the temperature (my cellar fluctuates between 12°C and 16°C).

When storing Port it is preferable to store the bottles in the same position as the previous owner. The manufacturers often mark the 'top' of the bottle with white paint, so if you see a paint mark / blotch, store this upppermost.