Every good fundraiser will tell you that one of the best ways to appeal to supporters is to appeal to their hearts so make sure you communicate your charities message clearly. Think about the type of charity you are supporting and then pick challenges that potential donors can relate to.
Most charities have iconic events throughout the year so if you are running for Clic Sargent do a sponsored dress as a nurse day. If you are skydiving for Barnardos, setup a baby sitting service. Think about your charity and how you can help supporters connect with that charity. This may seem obvious to some people but it is all too apparent that fundraisers do not think through the challenge they are taking on. Charities have spent years developing an image in the public eye so use their branding; wear their t-shirts, quote the charity if you are communicating with your network of friends.
Most of the running events that people take on have been done by someone else in the past so make use of the information available to you and if you are taking on a new challenge be sure to document it for the next person to benefit from.
Once you have formulated your plans read everything you can about your charity, the easiest way is to visit their website and get cracking. There will be a host of historical information there for you to look through. You are technically working for the charity for the next few weeks, months so make sure you could pass your job interview. If you are able to talk passionately about the cause you are supporting, people will sponsor you and the road to a sponsorship target will be a very steady one.