Quick Tip: The most important thing about watching a sporting event in the rain is that everyone in your party has a positive attitude about it.
If the sporting event you are going to may or may not get cancelled by the rain, make sure to have an alternative event planned. If you going with your family, take them out to lunch or dinner in the town or city that you travelled to instead. This will brighten the mood. In the case of my girlfriend, Well Dressed Sports Gal likes going to sporting events when the weather is nice, but her fandom doesn't extend to rain, sleet or snow. Fair enough. Take a buddy to the game instead and have a nice evening at home with the ladyfriend afterword.
If your traveling partner is game for traveling to the event, be prepared. A simple store bought can of waterproofing spray will go a long way. Bring a waterproofed rain jacket. Hopefully for you, you've already waterproofed your favorite cap just in case. If not, shame on you, a can of this stuff is only six bucks.
Quick Tip #2: If you plan on bringing an umbrella, check on the venue's website to make sure they are allowed in the stadium.
What I've got my eye on: I just purchased a custom made college football jersey. Here is hoping these become a trend. I'd go with the commemorative World Series Champions Phillies t-shirt or sweatshirt rather than a cap. Why? You can pass it on to your kids and it might be a while until your squad wins another world championship (or your city for that matter Philly).
A line of vintage t-shirts at Ebbets Field Flannels is really making me salivate. Check out this San Francisco Seals longsleeve shirt. Also available are the Rosewell Rockets, Denver Bears, and Jersey Giants. You can wear these under an open zip up hoodie for maximum effect.