Mark's suggestion of putting a plate directly on top of the monopod is cheap and stable, although not very flexible. But it works well if you're shooting stuff on the ground - so most sports.
Using a heavy lens on a monopod with a ballhead requires a special technique. You need to hold the lens with one hand, hold the monopod with another hand and tighten the head with the remaining hand.
Tilt heads work really well. The 234RC2 is wonderfully cheap but it is very droopy when used with a heavy lens. I use the RRS head, which is amazingly stable but very expensive. As mentioned in the other post, Custom Brackets do something similar for a much nicer price, but I've never tried it.