Most of the previous posters already gave the same advice that I was thinking of. Here are some other points I was thinking of:
(1) Don't spend money you don't have or really need (like for rent, food, etc).
(2) There are thousands of flippers/investors trolling ebay and the auction house listings every day trying to make money. You need to really educate yourself by using this board and other resources to do well. If you find this stuff fun, which you might well because you collected as a kid, it will be great. However, if you find it boring, tedious, and too much work, you'd be better off just parking your money with a good financial advisor.
(3) Your buying price is key. There are a lot of things you can buy, but if you purchase at the wrong price, it will be tough to re-coup your investment. Buying at the wrong price can mean buying really hot items when they are at their peak value or just buying an item higher than everyone else is paying. This is why knowledge and using resources such as VCP (vintagecardprices) or completed sales histories needs to guide your buying. Here is another useful article from a recent SCD: (here:
Link) on another iconic card that is popular for investments.
(4) A corallary to #3, patience, patience, patience. The card you want will in most cases inevitably come around again. Make sure it's at the price you want, in the condition you want, with the eye appeal you want.
(5) As others have said, it's still supply and demand. It's better to buy cards that others also really want, and one that has limited supply. Most on this board, including myself, are strong proponents of absolute scarcity/supply as opposed to grade scarcity, but there are others who have different opinions. The main reason for absolute scarcity is that grading scarcity can be subjective. A Gem Mint 10 in one grader's eyes is a 9 in another, and that can mean tens of thousands of dollars in value for the most sought after cards. That's too much for us to put our faith in.
Good luck whatever you decide to do.