Football, baseball, soccer, hockey, basketball, gymnastics, skateboarding, golf.... here are some ideas for planning your next sports themed party.  All of the ideas here can be modified slightly to coincide with the sport that you choose.  Scroll down for each sport's department.

  Invitations

You can purchase premade invitations (see the right column for ideas and links) or you can make your own...
Be creative ~ for a baseball party, cut out a circle and sketch 'stiches' with a pencil. Add the wording and party info with a black fine line pen.  (or you could also cut out 'bases' or uniform t-shirts, a baseball mitt, etc... You can also buy pre cut - die cuts at your local scrapbook store.  You could use wording like ~ Don't strike out!  Join the team for Jason's 10th Birthday!  This is when your "sports nut" can help you out with coming up with sports related wording...here is another idea: Hey sports fan!  You have been 'drafted' to participate in Colton's 10th birthday! 

Here is an example of the GREAT sports-themed invitations from Fine Stationery. 
Click above for details.

Decorations

Hang pennant flags from wall to wall.  Big inflatable '#1 hands' (see photo at right). For football, construct a large goal post out of cardboard tubes that wrapping paper comes on...Paint in the colors of your choice.  Gather all of the related 'sport of choice' items that you can find to display all over. Use helium balloons, tablecovers, plates, cups, and napkins in favorite team's colors.

Stadium fare

Of course, hot dogs, hamburgers, potato chips, pickles, peanuts and canned soda or gatorade sports drink.

You can purchase the white paper holders (the kind that they use at the stadiums) at Gordon Foods, Sams Club Warehouse and the like.

Activites

Of course if this is a basketball, baseball, football, themed and you have enough guests to play and a good place to play-- you could "Play Ball"!

Have the party guests each pick a sports nickname for each other.

Ahead of time, using posterboard cut pennants out for the party guest to decorate with their favorite team's colors. Just provide markers and such...

Play hot potato ~ only with a baseball, hockey puck, etc...

Play a sports trivia game -- Purchase one or make one up with index cards...
  you could make it only hockey related or golf related, etc...

Party favors

Stadium cup filled with foil wrapped chocolates, etc...  wrapped in cellophane and tied with matching curly ribbon, with a little pennant flag with the guest's name and sport's nickname or number.  You could also fill a plastic sports bottle (found at the dollar store) with gumballs.  Personalize each bottle with each guest's name.