How Cards Grow In Value

Cards hold their value for various reasons.  Supply/Demand is the obvious primary reason.  Lots of factors can affect both supply and demand.

Supply
- The supply of cards goes down over time as they age, get lost, leave the market (collectors and speculators), get destroyed etc.  This is normal for any collectable.
- WOTC can reprint a card, increasing supply (note that this increases a card via a new printing, which doesn't increase supply for previous printings).
- WOTC can also put a card on the reserved list, promising never to reprint the card, thus restricting supply to those cards already printed
- WOTC can also set the rarity, or do other weird things like promo cards, or buy a box cards, or sell cards in specific products other than booster packs, which also affects supply.
- and finally, yes, Chinese can counterfeit cards, thus polluting the market and increasing supply

Demand
- Demand is more complicated.  The obvious first part is how "useful" the card is in gameplay.  Powerful cards will be highly sought after for the respective formats, and thus that will increase demand.
- WOTC can ban cards from certain formats, and cards naturally age out of standard into modern and other formats, and this will affect demand.
- Other cards may be printed which will suddenly make combos with existing cards, spiking demand as people want to create those combo decks.
- White vs. Black border people will demand cards of the specific border.
- Foil players will always want those cards.
- Collectors will always want cards, either in singles or full play sets, to fill in holes in their collections.  This is likely going to be true well after WOTC ends MTG
- WOTC could end MTG.  This is very unlikely as the player base and sales have been on an upswing, and MTG has weathered storms before (homelands etc).
- The condition of the card and its printing will also affect demand.
- The price of a card will also affect its demand.  High quality Alpha's and Beta's are now so rare and expensive that they may have hit a ceiling for real demand, which explains why unlimited and now revised are so highly sought after.

As you can see, it gets complicated quickly, and it can be quite difficult to predict what the value of a card will do.  Generally speaking however, my advice is as follows for cards you've just opened from a new set.

1) If you aren't going to play that rare, and it's price is high (> $5) I would consider selling it a few weeks into the release.  Do NOT sell cards at a pre-release, this is a bad idea since no one generally knows which cards are going to be the heavy hitters and I have sold cards that then spiked in the following weeks.
2) If it's a high priced land, its value will likely remain high and probably increase over time.  Lands are typically solid investment cards, especially foil lands.
3) Standard cards generally go down in price over time as they get closer to roll out to modern.  There's simply more standard players generally speaking.
4) Reprinted cards may cause a spike in value for that card because it's now legal for standard, this is a good time to sell the older printings of that card.

I think that's all I have, I hope that helps.