I suggest growing a legume you like--or a succession of them. You could start with beans and then peas when the days get very short. If you like fava beans, those are very useful--you can eat the tender shoots and leaves, and they make big beautiful plants with pretty flowers that look so cheerful in the coolest months (but they take a LONG time to make beans!). When the beans/peas are done, leave their roots behind in the soil. They'll add a little nitrogen, though not as much if you let them grow to maturity. You will still need to replenish other nutrients with compost or a good tomato-specific fertilizer in the spring. One thing to consider is that tomatoes catch a lot of diseases that accumulate in the soil year after year, so that might be why you have trouble when reusing the soil. You could try solarizing any infected soil by covering it with clear plastic in full sun for a few hot months (March through June minimum), but that's tough when garden space is precious!

- colleen