I've been at my new job for almost 6 months. At the end of every day, I write down a quick list of everything I worked on that day.
I have Outlook set up to popup a reminder about 15 minutes before I normally leave. Then I fire up Evernote and, for each project, write a short bulleted list of everything I did.
Sometimes I leave early and I forget to write up the log entry before I go. But I always write the log entry for the previous day as soon as I come in in the morning. It took some time to get into the habit of doing it but having this log has saved me a ton of time and work.
We have to send weekly status reports in every Monday for the previous week. I just copy all of my daily log entries for the week into an email and send them along.
Have you ever wondered when you implemented something? Or when something changed? Or when something was pushed to production? Just this morning, I needed to find out the exact day that an update went live.
It doesn't matter how you do it. Send yourself emails. Use a software tool. Write it on paper.
But keep track of what you're doing.