One of the first things that a Christian has to learn when dealing with fen (SF Fans) is that many of them are openly hostile to Christians. This fact gets worse if the Christians behave in a manner that feeds their stereotype. This makes some methods of evangelism, such as "street corner preaching" and tract distribution destined not only to fail, but to strain relations between the Christians and their fannish friends. So, even if you see SF fandom an unreached community, you must be very careful how you practice evangelism in the fannish community.
Instead, the two purposes that should be the most seen in the fannish community are Service and Fellowship.
Service in fandom can be fairly easy. This is epically true in convention attending fandom. Most cons are run by a dedicated, but often overworked and exhausted, group of volunteers. At any convention there are numerous opportunities to serve, which will benefit the convention. At one end you can volunteer to join the convention organization committee. Depending on your position, you can end up working for several hours during the convention, or you can spend a couple of days worth of hours over the year or years before the convention. If you ask, I'm sure that most conventions can, eventually, find a position where you can shine.
Two other things you can do with less commitment are agree to be on programming, and formally volunteer to be a "gopher." Programing people are always looking for people with different perspectives to put on their panels, and most conventions need at least as many hours of gopher labor as they need from the staff or con committee (concom).
A final way is to just pitch in and do what needs doing. In 1998 I attended Copercon in Phoenix. Looking for a place to sit and kill some time, I wandered into the room where they were setting up for masquerade. Quickly I realized that they needed another set of hands in getting the stage draping assembled, and I jumped in. Without ever officially volunteering, I ended up helping setup and later tear down the draping, and even carried it out to the truck which would drive it home.
Fellowship may seem a bit trickier, but it isn't. Chances are that there is at least one Christian wondering around any convention wondering "am I the only Christian here?" At a small convention the answer may be "yes," but at most the answer will be "no." So the way to support the purpose of fellowship for Christians in the fannish community is to provide a place where other Christians can gather. I have seen this done in three ways, each of which has a different effect, and each of which can touch on other purposes.
First, if you have contacts within the people organizing the convention, see if you can set up a Christian Fandom meeting, or a Christian "Birds of a Feather" gathering as part of the programming. Larger cons, such as Worldcon, will often have standard arrangements for Special Interest Groups of Birds of a Feather meeting. The Christian Fandom list has often discussed how to have a Christian Fandom meeting. You can find out more about Christian Fandom and the list at http://www.christian-fandom.org.
Second, you can hold a Christian Fandom (or simply Christian if you don't want to associate with the Christian Fandom group for some reason) party at the convention. If you hold a Christian Fandom party, it needs to be a publicized open party that any member of the convention can attend. In addition to attracting Christians, a CF party will often attract other fen who want a safe party. It may also attract seekers. If this happens be prepared to discuss their issues, either privately or in a small group or with the entire party. You may also attract hostile fen. You need to treat them kindly and with respect, meeting their challenges as Jesus met the challenges of the hostile religious leaders of his time.
The only rule about a Christian Fandom party is that there should not be tracts lying about. Beyond that there are two traditions, a "loves and fishes" dish -- which is often a bag of goldfish crackers with a loaf of bread, but can be more, and chocolate.
The final way I've seen fellowship encouraged at cons is by holding some sort of a worship service. Depending on your beliefs and those of your local church, this may be best done by an official minister. In addition to encouraging the Christians in attendance to fellowship together, this also helps them to remember to worship.
I am the chair of ConSecration I (see our web site at http://www.consecration-con.org for more information). My vision in this regard for ConSecration I, and the subsequent ConSecrations that I hope will follow, is that they will be a place where Christian Fans can gather for fellowship -- both with Christians and non-christians who share similar interests -- and can serve and be served within the Christian and fannish communities, and for at least some be equipped to fellowship and serve Christians and non-Christians in the general fannish community. I also hope that ConSecration will help non-christian fen understand that there are Christians who are part of the fannish community, and that the Christians enjoy the same stuff, and do the same things as they do.