I've never done it but I read that 'hairy vetch' is a good option. Some kind of legume.

I have used hairy vetch once; I had to order it from a farm supply company and it came in large quantity, like per-pound. I used it in a meadow that we were transitioning from scrub/weeds to wildflower borders surrounding turf grass. It worked and boosted the nitrogen quotient significantly, but I gotta tell you, it was a hairy beast to till under (though to be fair, I was working a large area as opposed to a contained bed).

The thing I personally love about raised beds in the first place is avoiding work like double-digging, so I have an alternate suggestion. In late winter, a couple of weeks before spring breaks where you are, cover each of your beds with heavy duty black plastic, weighted down with whatever you have on hand, rocks, bricks, etc. Leave it on there for a couple of weeks and it will do two things 1) discourage weed seeds from germinating, and 2) warm that soil right up! When it's time to plant your spring crops in that bed you can either cut holes in it and pop the seedlings right in, or remove it altogether.

Doesn't solarization also kill a lot of good bacteria in the soil? Why not just plant a patch of late peas and when they play out, cover them with cardboard and compost/mulch and you'll be ready for next spring? It is a raised bed we're talking about. Shouldn't be a difficult undertaking.

Thanks for the suggestions. The late peas is a good idea. I was reading somewhere planting blackeye peas and leaving the plants would work. I will be trying that method this fall, to plant them and leave them in. The peas add nutrition to the soil too :)

susan2010, good point; true solarization kills beneficial organisms, but in order to actually solarize soil, you would need to leave the plastic on for a much longer period of time than the 2-3 weeks I recommended. Also the fact that it's in the cool season means that it's not going to heat up all that much--just enough to get the soil a head start on spring.

I wouldn't just leave the plants. I'd do the cardboard or paper and mulch over them in the fall. That way the bed will have all winter to decompose the stuff and be really ready in the spring.

You could try winter wheat. Another option would be to allow crops like spinach, kale, etc to overwinter. They may slow down in the winter and then start growing again in the spring. Once you are ready to plant (or before it bolts), pull the plant and put your new crops in it's place.