I've been keeping a technical diary in the form of an intranet blog for some time now and it's been working quite well for me. It allows me to record my daily activities, interesting links and project experiences.
What I find useful is that the blog engine makes it easy to search by tag, category, date, or keyword - so even if you have multiple projects on the go you can tag/catergorize them as you like. Also by making the blog available to your co-workers they can gain the benefit of your notes as well, and even add comments/further suggestions.
There are plenty of blog engines out there which are free/open source and extremely easy to setup (i use BlogEngine.Net along with Windows Live Writer)
If you are not keen on the idea of other people seeing the blog then I recommend using Evernote which is an awesome note keeping utility.
I guess the key thing is that you have to make it as easy as possible to use, otherwise you won't do it for very long :)