Card values never remain static for long. Typically, large swings in value will happen after States, Regionals, World Championships, and the first PTQ or Grand Prix of a new season. Before these tournaments hit, you need to be prepared for the change in value. Prepare ahead of time, and try to figure out which decks will win, trading for those cards along the way.
Look to these tournaments often so that you can make sure that you have a good picture of what’s going on. What you’ll see is that typically the decks that do really well are going to be the decks that everyone expects, albeit with a few minor changes. Mark down those changes, and adjust the cards that these pros chose to put into their decks a small bit up in value, while those cards that they took out of their decks need to be marked way down in value. If a new deck appears, make the previously undervalued cards move up a point or two, unless that deck does extremely well (one top 8 or more). In that case, make the card a seven or an eight on your scale, and be prepared for heavy demand.
Minor changes will occur throughout the season as well. New articles which are posted on all of the major websites need to be considered, and the rares potentially revalued depending on how many people you think are likely to play any given deck, even if the article is really bad. Also measure the amount of exposure that an article will receive. While there are some pretty good articles on some websites, if they’re not well known, not many people will read them.
Other changes may need to be made in the case of a very powerful uncommon. In the case of cards like Astral Slide, Fact or Fiction, and Tinker, those cards can have the value of a lower-middling rare depending on where you’re trading.
The person you’re trading with also needs to be considered, and this means that you’ll need to alter the values of your cards on the fly (always moving them up of course). If they are trading you for all the components of one deck or one color, they’re probably very willing to trade away their stuff which belongs in a different deck or color, so you can get those for a good value. Also note that people like this love foil cards, so if you keep your foils separate, offer to show them that collection. If the person wants stuff from all over the place, you’re not likely to get a really good deal from them, but if they spot your chase cards, make them pay. People who try to get everything for every deck are going to have a greedy eye for those chase cards, and most of the time, they’re willing to pay to get them.
Location can be important. Tournaments tend to be full of like-minded players. Get an overall feel for the quality of player, and if the field is mostly made up of casual players, your casual cards had better jump a point or two when you’re trading with them.
If you revalue your cards every week or so, you’re probably doing too much work. If you keep your card values static throughout the course of a season, you’re doing more harm than good. Our team made a conscious effort to revalue our cards at each major juncture throughout the season, and minutely configured the specific values of certain cards whenever we saw a major change. Overhauls are not necessary so long as you do a good job of valuing your stuff initially.