I think it is important to understand yourself, and how you function. This is something you learn over time and we are all different. What works for one may not work for another, although tips from others can point you in the right direction.
I find that trying to force ideas when they are not flowing seems make things worse.
A relaxed mind is important too. Stress, negative emotions etc can all add to the problem.
I also find that it is useful to note down ideas when they do come. I have many ideas noted down that I have not yet fully developed, however as I get additional thoughts on those ideas, I add them in. I often start a new document on a new idea, and work on it slowly as I get a few thoughts about it. That way, I have multiple topics going at any one time, and if I do not feel like writing about one topic, I can easily find another that I have already started something on.
I have a hub about Writers Block that you may find useful: