Let me preface this question by saying that I am in favor of collecting for fun and not necessarily for profit.

Stamps are like any investment: stocks, bonds, coins, art, rare metals, etc. If you're going to invest in stamps, you need to take time to study the market, watch trends, learn values, know when to sell and buy, and so forth. You can't sink money into the stock market, ignore it, and expect your investment to grow by magic. It takes knowledge, time and planning for your investment to grow. Stamps are no different.

When it comes to US material, the pre-1900 era stamps have a good track record in the long term. Generally speaking, mint stamps have done better than used stamps. Condition and centering are very important. Stamps that are VF or better in centering and without faults perform considerably better than stamps that are off center and/or damaged (thins, tears, etc).

If you're going to invest in stamps, be prepared to spend a sizeable sum of money and be prepared to hold onto your investment for several years at least. Yes, there are a few instances where stamps have shot up in value in a short time period. But the vast majority of stamps appreciate over time. And in general, you're talking about stamps that sell for $100 or more. And often times you're talking stamps selling in the several thousands of dollars range. You're not going to invest in stamps that catalog 20 and 50. Common stamps just don't grow in value very quickly.

Investing takes money, time, patience, and a lot of work. There are a few brokers in the hobby that have made a business out of investing in stamps for customers. You may want to consider using a broker (which usually involves a fee). Even so, using a broker is no guarantee that your investment will grow.