I personally have no love for publishers - I am aware that they wring the necks of developers fairly hard, and (as with retailers) are profiting from someone else's work. Sadly, it is the system in place and the only main form of supporting developers is to go through retailer and publisher channels.  Its a twisted, sick system that I do not agree - its pure capitalism and a method used throughout any system of selling products and goods. I try to support developers that sell direct and can only hope this form of distribution becomes commonplace. For starters, all monies go directly to the developer and not through some convoluted corporate hierarchy and, secondly, the prices are significantly reduced when publishers and retailers aren't adding their own expenses and profits on top.
 
However, within the current, prevalent purchase model available, buying first hand but waiting a year to purchase (when the price drops) or buying second hand doesn't help the developer. 
 
Developers operate (in the main) on a game by game basis. If their current game flops, then they may not be able to finance their next creation or may find it hard to find backing from now wary publishers. By the time a game has dropped in price some 6 months to a year later, the developers fate is already sealed. Whatever monies and contractual arrangements procured from the release will have been and gone. At this juncture, the only benefit is to the retailer, who is trying to shift surplus stock.
 
The arrangement between retailers and publishers runs as thus (and you are probably aware of this, so forgive me if I am teaching you to suck eggs). The publishers invest money in advertising, drumming up hype for the game. The retailers, based on this hype, will order in stock. The retailers end of the bargain is a risk / reward structure. They buy in stock based on projected sales and then either shift them for profit (retailers add a significant mark up on titles - they buy a game in for £10 per unit and then sell it at £40 (I'm in the UK btw)) or get lumbered with unsold goods.
 
When stock is bought by the retailer, that is when the publisher gets its money. As far as the publisher is concerned, if a retailer buys, say, a 300 copies of a game then they (the publisher) have sold 300 copies, even if the retailer fails to shift those 300. However, if that stock sells out, then the retailer (if the game has been popular) will buy further stock (say a further 200 copies).  At this juncture, the publisher will see that the game has now sold 500 copies. This cycle continues until interest in the game has died down and purchasing of the title drops. At this point, the retailer will not order any more stock and will try and shift whatever remaining stock they have, mainly through sales and special offers. But this is just the retailer offing surplus stock. As far as the publisher is concerned, the game has sold 500 copies, even if the retailer still has 100 copies on selves / in warehouse.
 
The amount of stock the retailer shifts will indicate to them the worth of investing in stock for a sequel to that game or a further game by the publisher / developer.  If a game sales badly upon release, and they are lumbered with surplus they has to shift at a reduced profit, then the retailer will be less inclined to invest in further titles. 
 
When the retailer sells a game second hand, if they are also selling the game first hand then they are doing nothing but lining their own pockets. The second hand market means that retailers needn't continue to re-order new stock as they can simply recycle their existing stock and continuously make a profit on each unit without the publisher seeing another unit shift. This is where the second hand market impacts the developers. If the publisher sells 100 units to a retailer and then the retailer doesn't buy any more stock in and simply re-sells that stock as second hand, then the publisher may view the game as a flop, even if the retailer has sold each unit 4 times over. At this juncture, the developer, and not the retailer, will become wary of investing in any further titles from the developers of said game.
 
So, buying a first-hand game upon release is the best way to support the developers. 
 
Finally, I bought both Blur and Brutal Legend and yeah it sucks that they dropped in price so quick, but its simply indicative of the issues laid out above (they didn't sell well). At least I can rest easy knowing that, if those companies fold (which they are not, thank god) I wasn't to blame for it!