I would suggest a good f2.8 70-200mm lens. At 105mm it is perfect focal length for portraits and it offers a little zoom for those football games.
If you're willing to spring the money then I would also suggest a good f2.8 400mm as well. (These lenses will cost quite a bit of money) such a long zoom is good for the side line distances. A VERY good investment would be a monopod. holding a camera steady should not be your biggest concern when trying to shoot. I am not very familiar with pentax lenses though so The availability of aforementioned lenses is questionable.

technique: try to follow the stars. People you think will be running the ball. High ISO's are suggestable (around 1600). try to keep your shutter speed up around 1/250th of a second. I'm not familiar with the ist D camera so I don't know how fast the autodrive is. If it's below 5 fps I wouldn't monkey around with it though. Just try to predict when the peak of the action will be and get close with a long lens. If you can predict the action precisely and shoot it consistently you will see great results. eventually once you've mastered the basics you can begin to concentrate on shooting what tells the story of the game best (example being a shot of the underdog getting hit hard etc.) good luck and happy shooting.