As I recall reading a long time ago, there are four different levels of aircraft storage available:
1) Extreme short term - the least expensive, but very little preparation is done. I believe this is for less than one month's time.
2) Short term - leased jets undergo this routine frequently. An airline finds itself with too much capacity, and decides to return the planes to the company. Sometimes the leasing company can find new customers right away, sometimes not. More preparation is done for this, like covering windows and tires and engine intakes.
3) Long term - this was a popular decision after 9/11. Airlines shed their excess capacity by storing a lot of planes (especially wide-bodies) in the desert. When it is believed that the aircraft will not be needed for a long period of time, extensive preparations go into covering and securing the plane from wind, sand, moisture, etc. This particular practice requires a LOT of man-hours to cover and secure the plane, but when the plane is needed again, it will be in great condition and require only minimal replacements of parts. Usually it will be in flyable condition in a very short time - witness the AI
777's taken quickly from the desert.
4) Retired - this is where planes go to die. No preparations are undertaken as the plane is not expected to fly again, merely cut-up or stripped for parts.
In answer to your question (finally!!), the techniques used by storage companies are quite refined - I can't say "indefinitely", but I would say the more money you have to spend, the more secure your plane will be.
As a side note, I have read that the cost of the most expensive level of secure storage per day is still cheaper than the landing fees at some airports!
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