My way of keeping an art journal is to share my WIP's (work in progress), something I started to do on art forums a long time ago. At the time I did it for feedback, but I found that it's a good way to keep track of formulas, experimental successes and failures and the whole thought process. So now I keep a WIP page on my website as a way to share my art and what I've learned and to always have a record I can refer back to.
I think it's important to challenge myself with each new piece...sometimes it's just a conceptual challenge, sometimes it's trying or re-cycling a new technique. I can't always remember an obscure color formula or the order of certain steps but I always remember which pieces I used a certain technique on, so it's easy to find the information I need by going back in the archives to that piece.
Yesterday I started a new sculpture and I'm chronicling the process. I do this for all my work, though the less important practice pieces sometimes end up on my onsite blog rather than as a detailed step by step journal since they aren't all that involved.
I find it easy to keep updated since I formulated a blog type form for this page so I can just pop in and add a new entry. It also allows me to control how many entries appear on each page and it automatically creates a new page.
I size my photos to a maximum size of 325 pixels...I keep the quality high but it makes for a small file size and easier loading.
It's not hard to record the process...I just basically go step by step and describe my thought process as I go. I'm not one of those people that sketches out their ideas, it's already firmly embedded in my head, or close enough that I just have to work out a couple mechanical issues as I go. Plus unless you're going to cut out and file each sketch page it's going to be pretty hard to go back and find what you need. You don't need to publish your work in progress page if you feel awkward about it...you can just go to your Word or NeoOffice program, write and insert photos and then save into a special file to refer back to.
I also take a lot of photos of things I find interesting and I also download photos to use as reference. I'm not copying them but using them to understand details or get conceptual ideas. I categorize all of them in my iPhoto program so they're handy. This may not seem like it's a technical art journal but color, masses and shape are important to my artwork and may be the answer to a technical issue I'm having. So, again, I've given myself a place to research when I need anwers.