I'm not sure how many hives you have, but here's some inexpensive things that I routinely do to strengthen mine. It has worked great so far.
1. Swap positions in the beeyard with a weak hive and a strong hive, in mid-day. This is when most of the foraging bees are out of the hives working, so place a weak hive that needs more bees where the strong was, and visa versa. This way this small weak hive will have an instant influx of worker bees. Bees covered with pollen and nectar are accepted in the weak hive, even if they are not FROM the same hive.
2. Take capped and/or uncapped brood from a strong hive and add (or swap frames) with a weak hive. This gives a bolt of brood when these hatch. I ROUTINELY take and give from all my hives, to any hive that is in need of something....(i.e. honey, pollen/nectar, brood, empty comb, etc...)
3. Rotate the brood chambers every 3 weeks to provide the queen with a constant supply of open drawn comb, to stimulate egg-laying and encourage brood production. (i.e top on bottom, bottom on top)
4. Feed sugar water to make up for what they don't find available locally, at the given time.
I'm going to experiment this winter and build a heated container for 5 of my strongest hives, and actually maintain heat for these five hives, in an effort to keep the hive building brood all winter long. Hopefully, I can make some multiple splits for next spring and get a jump on 2008. It's going to be something very simple, and just maintain the temp so the bees don't ball, and the temp doesn't drop very low. Hopefully it'll boost my splits and queen rearing for next year. I'll post and let everyone know how it goes.
Best of Luck