The first rule to collecting is there is no rule. Collect what you like and if you want/need to flip cards, and are successful at it, to help fund the hobby it certainly can help off set the costs of collecting...which can be quite expensive depending on your collecting focus. Study the market, understand what things should sell for roughly in certain grades, and use the resources at your disposal to track sales/trends in the market. If you can afford it, try to focus pick-ups to HOF'ers and always buy things you can afford and would be happy to keep in the event you can't sell quickly or the market changes. Don't over extend yourself financially, as already mentioned but, as with most things in life, a little calculated risk can sometimes yield the greatest rewards. Just do your research and remember that sometimes the best deal is the deal you don't make. These are just general things that I always try to keep in mind, and they seem to serve me pretty well.