In my case, I am really in a niche market, so it is pretty easy. But even if you are let say in general iOS App development you can find the commonalities in your work which can help others in the future.
Let say you created a nice messaging system for an App, you then explain that this expertise can be reused from messaging between people on a human disaster zone but also when people are meeting to run a "catch the flag" game in your area.
Nothing crazy, the real thing is really, you sent a letter, a real one, you tried to wrote a small note reminding the contact about a good time together or wishing success in the area he is working (be just specific enough so he knows it is not a "I write the same on all the cards" note) and simply try to be nice.
Let say you created a nice messaging system for an App, you then explain that this expertise can be reused from messaging between people on a human disaster zone but also when people are meeting to run a "catch the flag" game in your area.
Here is for example my link at the back I sent this year: http://ceondo.com/2015
Nothing crazy, the real thing is really, you sent a letter, a real one, you tried to wrote a small note reminding the contact about a good time together or wishing success in the area he is working (be just specific enough so he knows it is not a "I write the same on all the cards" note) and simply try to be nice.