I agree with all the above. While invariably not everyone that you deal with (sadly) will be using the GTD methodology, you can surreptitiously impose a GTD approach on them - ie, help them work out what the very next action is.

In terms of frequency of follow up, I've built 'follow up my waiting fors' into my weekly review. I don't necessarily follow up each item each time, but it means I have a weekly sweep of that, and then it's out of mind in one go.