If you're going the flash card route, then I thoroughly recommend an SRS (Spaced Repetition System). It has helped me immensely in the retention of my Japanese knowledge. I haven't entered many cards for learning programming yet, simply because I haven't found the need. I would imagine for more OS specific concepts or function names it would be very useful to retain some of that knowledge. I have also found it useful for drilling mathematical equations and techniques, with I am very prone to forgetting.
A word of warning would be that you only want to use an SRS to quiz yourself on knowledge that don't yet know, or are failing to retain. Adding everything you come across is very tempting, but the stress of having to maintain a daily rep count soon takes its toll.
I use Anki (https://apps.ankiweb.net/
), which is starting to show its age now (and the mobile version is expensive and slow). However it does has some nice features. If I had the time I would build my own because it's a very simple concept that no one seems to get right.