I say sell it all. It's time to think about cashing in your chips and placing your bets elsewhere, there's simply no more money to be made off these cards. The longer you hold onto em, the more you're bound to lose.
I am, of course, referring to those Theros block staples you've been sitting on for the last year (note: I'm lumping M15 in with Theros, as they rotate together). With the lack of clearly defined PTQ seasons, rotation is not as clear as it used to be, but the writing is already on the wall and understanding the halflife of Standard prices is critical for maintaining your collection's value as a player.
As a general rule, your cards are always going to be worth the most money when you are most likely to need them... and there's a reason for that. Cards that appear in popular and successful decks tend to hold their price because people need them to win, and I'm guessing that you're probably trying to win once in a while.
For the bulk of Standard cards, rotation means a financial flatline. Even Standard staple all stars aren't immune to this effect.
The pricing graph for Thragtusk pretty much sums up everything you need to know about price trends for Standard cards.
First: the price peaks right after the rotation, in this case that would have been as Scars of Mirrodin block left the format. The price always peaks at this time because it's likely that the new set will hog the spotlight and the older sets just won't be opened, and there is usually a fundamental shift in the metagame at this time.
New metagame means new decks leading the format and players are left often left scrambling to acquire the best deck in a short amount of time forcing demand (and subsequently price) to spike before tapering off.
Second: It's pretty much all downhill from there. For many players, acquiring the most powerful cards is a means to an end. That end is (was) usually Pro Tour invites or large cash prizes. In the past, we had clearly defined PTQ seasons and needed those cards until our last tournament of the season.
With the PPTQ system, however, there is likely going to be a more gradual sell off as rotation becomes imminent. But, regardless of the rotation trends, there are very few examples of a card spiking after the October/November set release and metagame shift.
It's Time to Sell
Now's the time to take stock of your play style and your play needs for the next 8 months.
While you can hold on to your playsets of Temples and Painlands, it's likely you know what colors you're not going to have any interest in playing between now and August. If you're strongly in the U/B Control camp, hold onto your Temple of Deceit, but ship everything else. If you see yourself playing Abzan or Sultai down to the wire, keep those Temple of Malady and Llanowar Wastess but consider shipping your Temple of Triumph, Temple of Epiphany, and Temple of Enlightenment.
While you're probably going to get stuck holding a few losers from Theros, knowing your personal taste and play style will help you avoid being stuck holding the whole lot.
The other thing to keep in mind: if you're waiting for the prices to peak, you've already missed it and now you're solely in the business of damage control.
Those of you that are familiar with stock markets understand the concept of short sales.
For those that don't - you're betting that a stock is going to go down in price so you borrow a number of shares to sell now with the promise of replacing them at a future date. If you sell a stock for $10 a share today and then rebuy it for $8 a share when it's time to return those shares, you've netted $2 a share in profits.
You can perform a similar function with Magic through trading, and, to a lesser effect, selling.
For example: going into Khans of Tarkir there were many indications that the Onslaught fetchlands were going to be reprinted. I knew that in the long term picture that I was going to need these cards to play Legacy but had no tournaments in the short term so I shipped nearly all of my Onslaught fetches with the intent to rebuy after release at a reduced price.
The other factor to take into account at rotation is how much you value your time. I know that Mana Confluence is going to take a hit at rotation, but is the difference between it's price now and it's price in eight months going to be worth the effort of shipping and reacquiring these cards?
If you're the type to move most of your cards to vendors and buylists for cash, the answer is probably not. If you're an active trader at tournaments or through services like Pucatrade, you can store that value in better investments without paying a percentage to the house.
Personally, I probably won't bother with my Mana Confluences because I picked them up at their lowest price and don't think the price will shift dramatically enough to warrant the hassle.
But what cards are sure sells at this point?
The Temples are done for. These cards have made little to no splash in Modern and certainly won't see any play in Legacy. There is no "up" for these cards any more, and the floor is basically bulk. While they'll probably remain popular in Commander and casual formats for a long time, the supply in circulation is utterly massive.
Remember the pricing on the Painlands before they were Standard legal again? I'll give you a hint: $2 was generous.
Just look at Temple of Abandon. That graph has a pretty clear trajectory and holding holding onto a playset of these will cost you $8 at rotation, and this is likely the least painful of the Temples.
Now look at that price tag--a playset of Temple of Malady is currently worth more than the ENTIRE SET OF TEMPLES will be worth at rotation.
This playset alone is going to cost you $36 to hold on to. You have to take this reality into consideration when deciding whether or not just to buylist these cards. You can often get a hefty trade credit bonus that will help you flip these cards into safer places to store your value.
[cardimage cardname="Stormbreath Dragon"] [cardimage cardname="Elspeth, Sun's Champion"]
Two more cards that saw impressive peak prices that are now falling like kamikaze pilots from the sky.
Both cards saw spikes around Khans' release. Stormbreath Dragon is currently on a trend towards stabilization, but Elspeth is lower than she's ever been due to the imminent release of the Elspeth vs Kiora duel deck later this month.
Elspeth is following Jace, Architect of Thought right into the abyss of former glory. Stormbreath has an outside chance of becoming relevant in Modern, but is still going to continue losing value until rotation.
Definite sells here.
In the spirit of making things easy, I'll just go ahead and make a list of cards that have been popular in Standard that have little chance of holding value after rotation:
These are the main cards to look at when creating your strategy to stem the bleeding.
I feel that there will be a casual appeal for many of the Gods for a long time after rotation, and most of their current value is tied to casual appeal rather than constructed viability.
Thoughtseize has been an eternal staple since the day it was first printed and will remain an essential card for eternal players for years to come. Anger of the Gods is likely to see continued use in Modern and is unlikely to see much more loss in value.
Chord of Calling has already pretty much bottomed out in price and has a rich and storied history as a Modern staple. Lastly, Eidolon is popular in Burn strategies across both Modern and Legacy, which I feel drives most of its current value.
Xenagos, the Reveler is a card I feel will lose a just a little more value, but ultimately has a chance to turn things around long term. He's a Planeswalker and has an ability that only needs a couple cards printed to get a break in Modern.