Davey Shafik raised an interesting point in his blog: experts tend to direct beginners to the GTD book instead of providing advice. I think the reasons for this are

a) GTD cannot be explained in short. It's a complex (but very well structured) system. You need to read the book (and optionally listen to audio materials, read forums etc) to deeply understand the concepts behind the system.

b) There aren't as much experts as one might think. I think most people never "get it". David Allen said that he doesn't know too many people on the planet either who care enough about their time to follow the principles 100%.

Regarding self-discipline, David Allen said that people don't need more self-discipline, they need a disciplined approach. I'd suggest everybody to read his second book titled "Ready for Anything" first. It answers the "why" behind the concepts while the first book tends to focus mainly on the "how".

Another usually misunderstood concept is procrastination. GTD doesn't help to overcome it at all but helps to predefine tasks so you can constructively procrastinate instead of surfing the web. David Allen said that the most effective people on the planet are the biggest procrastinators. The weekly review is absolutely necessary because that's when you define your tasks to procrastinate with.

Regarding software and other tools available for GTD out there, I've tried most of them. Sadly all of them sucks. They don't follow the strict GTD principles. Their creators tend to have "better" or "more efficient" ideas than David Allen had during the 19 years he put into this system until now (at least they think so). 99% of the time those ideas turn out to be wrong or inconsistent.

If you are interested in one single advice, I'd suggest you to stick to what David Allen says and don't try to change anything. If something doesn't fit into the system, you probably need to restructure your thoughts so that it does. It's very important to stick to the GTD language. Think in open loops, not projects; next actions, not tasks. It sadly seems that David Allen uses the word "project" inconsistently (although he doesn't). It's better not to use this word at all: they are either 1-2 year goals or open loops. This all might seem to be nitpicking but otherwise you can't benefit from the clean edges that the system defines. If you follow the principles, it's always obvious where anything fits in the grander scheme of things.

If I had to summarize GTD, I'd say it helps cleaning up your environment using the bottom-up approach (from "what's on your mind at this moment?" up to "what's your life purpose?") enabling you to focus on higher levels of your life as well. "The longer your horizon, the smarter your moves."

GTD is not about work. It's about getting the most out of your life. If you use it only for your work, you get stuck at the level of 30,000ft (1-2 year goals) or 40,000ft (3-5 year vision). There are many parallel things on these levels besides your work and if you want to get the most out of your life, you should seriously consider discovering them! On the other hand, it's an absolutely good idea to start GTDing only with your work (as long as that's what takes most of your attention), then move on to other areas of your life some months later when work needs much less thinking.

Here's a small internal script I keep trying to run on my mind:
crank widgets until interrupt;
on interrupt dump raw input;
on confused do a review;

Finally, some tips for those who've been GTDing for a while:
Always define exactly 1 next action on your next actions lists for each open loop. Defining more leads to anxiety and those steps will change anyways. If you want to dump those next actions out of your head, handle them as project support material. Also, if you seem to have parallel next actions then you probably need multiple open loops instead (with multiple desired outcomes).

Hopefully these random thoughts help you but you can find many more in the second book which I mentioned above.