Project completion obstacles.

I've only recently starting thinking about some of my more complicated tasks as "projects" as defined by David Allen's GTD system/book. I think this has helped me immeasurably (that's a lie, I could totally measure it) by at least forcing me to break the process into smaller more manageable bits. My problem is that I'm not prepared to do it the way GTD suggests - recording the first action and then dealing with the next action when that's done (or at least that's my reading of it) - because I know that I would put off each step just a little which would add up to a whole bunch of procrastination. So I determine all the steps, their order, and which ones can be done simultaneously. This front loads the project work which is one of the big barriers but once it's done you'd think it would be easy from then on in. My big problem is that I don't have a tool or system built well enough to easily organize the individual tasks and then, in the midst of completing them, I come up with a better way of organizing them, or I remember a new task that I forgot to insert somewhere... What I need is a good seamless method/tool to help me manage all this and insert it into my work flow I already have (i.e. Remember the Milk). I keep looking but I have yet to reach the finding stage.