Being a "supercollector" of quite a few players, my advice is to collect within your means. If you are rich and have a money tree growing in your backyard, then just go for everything you see. If not, be smart about it, watch the trends and don't ever overpay for any card. Some great advices above, here's a few key advices:
1. Avoid new products unless you want to overpay. There is always great demand when a set is just released, that means great competition for cards and great prices if you want to sell, not to buy. Once the initial storm is over, a month or 2, prices settle down to normal prices. As mentioned above, this is probably the only time you will have access to buying low numbered cards because people are busting packs and finding them.
2. Avoid rookies because their cards are priced on potential. Like new products, people rush to collect new players based on the promise/potential they have shown. How many of them actually turned into superstars? or even just semi-stars? Be patient and wait a year or 2 to get an idea of how good these players actually are. So basically, I would rule out supercollecting any rookies unless they are really really unloved by the hobby.
3. Understand that you can never defeat the card companies, they will always out-produce you. Prepare to let those 1-of-1s go, even the abundance of /5, /10 these days, and printing plates. The checklist is ever increasing if you collect legends, but you can always pick some crap players who has a finite checklist.
but that is the beauty of collecting...
