I am getting more and more tired of the sell and buy thing.
In earlier times with Nikon DSLR or Leica M bodies it was every 2-3 years a new model came up. But if I see what happens in m43 land and even worse Sony A bodies...uuaah.
I am beginning to learn a few things about the importance of camera cases and such to keep all the gear in "mint" condition. The used A7r cams now being offered for sale make ashamed when i look at my sample. Even Sony would not touch it with a 10 foot pole if anything goes wrong under warranty.
Many of those guys would probably die if they saw the micro scratches on the front elements of my naked Leica lenses...
I find selling the least enjoyable part of the whole equipment ownership experience. So many buyers are extremely picky about minor cosmetic things and how it relates to their idea of value. Of course they're thinking about resale prices down the road to other gear collectors... For me, I just want to use the gear. Maybe it is laziness (referring to Jono's post), at least for me looking at my difficult to cull collection. One justification is "you never know when you will need it (again)."
Regarding Paratom's point about the current release schedule by the likes of Sony: an advantage is you can wait a year and buy quite current equipment on the used market at good prices, especially since used prices decrease with the drop in retail price as new products are announced and old inventory is cleared.
If I was 'invested' in, or interested in a quickly cycling system such as Sony, I'd wait until the end of the product cycle to pick up equipment at a discount, or buy the previous generation used. Someone else will have taken most of the depreciation hit.
Back 10 years ago my philosophy was different: buy new and sell it within a year to 18 months even if there wasn't anything new, though I didn't always follow this due to various reasons. In this time span you'll have had the benefit of full warranty coverage and the gear being pretty much like new and not yet worn out/tired. Maybe you'd end up rebuying the same gear, but 12-18 months later, it may have gone down a bit in retail price, possibly offsetting some of the loss taken on the sale of the previous kit. It's kind of like being on a continuous short-term lease cycle. You will always lose money on the purchase, whichever way you go. Flipping gear constantly keeps you equipped with something new and/or the latest, if you like that. The other extreme is keeping it until it's dead, which may mean skipping one, two or more cycles. But by then it's worth pretty much zero. You're either taking a smaller loss in a short period that is more manageable, or you're practically writing the whole thing off after some time. With the latter, you face a bigger, though less frequent expenditure to reequip, for which you should be saving money regularly in anticipation of the eventual purchase. Broken down over time, assuming the new gear pricing is relatively constant, it may be fairly similar in cost.
Of course there are variables that can throw this all off. For me it was going from Canon 1D to 1DII to 1DIIN, but when I got to 1DIII, the cameras were quite temperamental and I never felt in good conscience to sell them knowing they were not in optimum functionality (the AF recall which didn't really resolve performance issues and other problems my cameras had). With Canon the cycle had worked out reasonably because the cameras remained very consistently priced, until the 1DX. Due perhaps to the 1DX's delayed release, switch to FF and considerably higher price, the discontinued 1DIV seemed to hold value much better and was a popular camera for some time. Kind of a rebound from the 1DIII. But the price gap to the 1DX was considerable.
I also use Leica and I think they're a bit more difficult to predict due to short term history. The M9 probably distorted resale prices and perception of what those should be a few years later because it was such a hit product. Many waited a long time to receive delivery, also for lenses that were in terribly short supply, which opened up the second hand market to speculators and drove prices higher than retail. It likely reinforced the perception of the 'Leica investment'. Since then, though, I feel there has been considerable deflation. I refuse to buy Leica digital M at introductory retail prices and instead will wait about a year, at least. I bought my M240 as a demo 1.5 years ago at a considerable savings then. There seem to be a lot of these available now. Leica lens resale values on various forum B&S have softened considerably, too. I don't expect M240 resale to hold nearly as well as what Dan achieved with his M9. Already you can buy new M240s from Hong Kong for well under $5000 US and M-Ps at or just over $5000. The only way this would reverse is if for some reason Leica stopped M production or the next model is considerably more expensive. But I suspect they will continue at similar introductory price points.