I've been meaning to write this for a while. I've figured out a few useful little lifehacks. 
Firstly, I hate to say it but I've been a member of the Moleskine bandwagon for a while. They're actually quite neat. I like the large sized ones, but that's because I write a lot down. The small ones are too small to jot stuff in, so stick with the large ones. Ruled. I'd prefer it if the lines were slightly closer together, but I guess they're just about fine.
In the back pocket, I keep two things - a print out of Paul Graham's Writing Briefly. It can be read and re-read. It also contains an A4 page listing all the things I should have done but haven't. Some people call it a "to-do list". I call it a "oh shit, forgot to do that list". I'm unorganised, which is probably a very good reason against listening to anything I have to say on the topic of lifehacks and productivity.
The to-do list is created by Remember The Milk, a great online productivity tool. What's nice about it is that you can quite easily tag all the things you are working on. And if you use the in-built search engine, you can produce a "Smart List" of all your items. Every day or so, load that up, press "Print" and you've got a to-do list that you can take with you. It also works on mobile devices too. If you are building a web app, making sure you have really good print modes makes it easier to hack them in to your existing system.
Another thing I can't live without is co.mments.com. It's a comment aggregator. You add a little button to your bookmarks bar, and when you are browsing a blog entry that you want to keep track of, you click the co.mments link and it gets added to a page. Whenever a new comment is posted on that blog entry, the page is updated. It has an RSS feed, but it's not really worth bothering with. I just check the page every day or so. I try to bookmark every comment I make using it so that I can continue conversations with people.
When it comes to chomping on ideas, I use everything I can. Outlines, lists, 'mind maps' and free note-taking all work for me. I've got an outliner on my Palm Pilot, which is often useful - and can be exported to OPML fairly easily.
With regard to GTD, I use it, but I'm not religious about it. I'm also far, far from perfect in my use of GTD. From the folks I know who are obsessive about their GTD, I can see the benefits in using it, but I'm lazy and always fall off the bandwagon. In the last academic year, I had my task management really down to a fine art, and despite a few crises, I stuck to it pretty well.
I'm still evolving towards the solution.
