I have been playing a year and still have trouble remembering. Once I remember, I always check my friends who play and remind them if they aren't tagged. Forgetting can ruin your week.
The worst feeling is when u remember u haven't joined GDI just after u make ur first attack!! This happens to me 50% of the time.
A couple of sets ago I was surprised I finished at #6 without GDI, I wasn't online those few hours at the end, didn't like to see my country hammered with nukes ... I guess suiciders were bored and left the game at the end of the set.