I definitely get an itch for "a new and cooler" bike with fuel injection, and better gas mileage every once in a while (I have 2007 Ninja 250, but wouldn't mind Honda CBR250 or NC700X)...
And I'm also in the same boat: old bike has been bought with cash 3 years ago when I was in college and has all the nice accessories, while I would have to buy stuff for the new one, and some of the accessories I made would go to waste (like the fiberglass handguards I made just for my Ninja, because nobody seems to make anything winter-worthy).
So the trick I play on myself, is tell myself that I will not quit this bike until it has xx,xxx miles on it... In my case, I'd would love to turn 100K miles on my Ninja 250. Will that happen? Not sure, but at least it gets me through the seasonal "new bike itch," and has gotten me to put 27K miles on the bike so far :-)
And it keeps my money safe for something more prudent (I'm in my 20's, so I still need/want to save for a house, investments, etc., and I imagine that many of you guys don't have that problem anymore). I guess it's not as much about the bike for me, as it is about waking up every morning, and just getting on the bike and going for a ride. I don't even own a car anymore, because it just sat there doing nothing, and I hated killing it in such a slow and painful death.
That's what works for me.