For me, I've realized that there is a "critical mass" you can hit in almost every aspect of the hobby that will burn you straight out for a while.
One way I found over this, is to set all of your in depth projects with all the extra detailing and competition grade finishing techniques aside and re aquaint yourself with the simple joys of doing a kit or two straight from the box.
Nothing fancy and nothing over and above what the manufacturer gave you, you don't even have to keep the finished model if you don't want to.
Doing a kit straight from the box puts me back in that joyous state of mind that I was in as a 7 or 8 year old just starting out on the hobby.
The detailing thing, I find, is a real burn out and motivation breaker. I've had Revell's 1/32 MiG-21 on the go on and off for 5 years now. Most of the extra detailing and scratchbuilding was required. I had to take a real long break after scratchbuilding new landing gear bays and landing gear legs. Again I needed a break after correcting and scratchbuilding some glaringly inaccurate fuselage surface details. Its still not complete, but I'd have given up on it long ago if I hadn't set it down for a month or two here and there.
Those out of the box kits I did during my breaks have saved the MiG from my scrap pile.
There is also the issue of overloading yourself with unbuilt kits, a few years ago I had about 20 or 30 unbuilt kits that I was staring down and getting to realizing tha tthere was no way I was going to get them all done before I lost intrest in building them. I picked a few to keep (no more than I thought I could complete in a two year time span) and gave the rest to a local thrift shop.
Not only did I get a load off my mind in that act, but I've probably given some enjoyment to a less fortunate kid or two, that when you look at the cost of kits today, wouldn't have a chance at the hobby otherwise.
A few rules I've made for myself to protect from burning out:
1: Know how much time you can reasonably give to the hobby and never stockpile more unbuilts than you can keep interested in (If I know I can't get to it in two years from time of purchase, I don't buy it).
2: Buy picky. Yes, it would be grand to have every kit that caught your eye for a split second in the hobby shop, but its not very realistic. Know what you like, know what you want and make real sure you WANT it. There's nothing worse than buying something then asking "why did I buy THAT?" a few months down the road.
3: Detail, how much do you really need? what can you justify and whats just frivolous waste that no one will see once the fuselage is glued together around it? Don't waste valuable time and materials on details that only you know are there.
4: NEVER underestimate the power of the straight from the box building experience. There is something very gratifying in having a beautiful model on your shelf and being able to say that there's nothing extra on it.
5: Keep it FUN! If it ain't fun, you're doing it wrong! Fun is what got all of us into this hobby and fun is what will keep us all in it. If that means we don't break the bank on aftermarket for every model we build or spend bizarre hours scratchbuilding something smaller than the eraser on the end of a pencil, so be it.
6: Build for YOURSELF first, forget competition judges, forget what anyone else thinks. Does the model satisfy YOU? If yes, then motivation will only be a minor issue for you, if no, adjust your attitude to the hobby. If you aren't doing it for yourself before anyone else, you're doing it for the wrong reasons.