One problem that I have faced over the years is that when I stop using a technology (COM,QT)/language (VBScript)/feature (Templates) for development over the time I lose skill in that.What in your opinion is the easiest way to retain familiarity so that when I come back to any of them, effort for relearning is minimal.
How to retain familiarity with previously worked on technology/language/feature
self improvement;skills
Keep hold of your own reference books. That way, you can go back and refresh your memory.  Personally, if you are not regularly using some memory allowing it to fade and to use that capacity for current stuff seems to be a more efficient use of the grey matter.  Rereading the same books that you originally used allows old memories to be refreshed surprisingly quickly.I am looking forward to 10 years time when COM programmers make 2000 a day.   :)