Highlight interrupt powers so you can easily scan for them when your creatures are hit/bloodied/etc.
I print up my own stat blocks. I copy and paste the text from DDI into my notes. Originally I did this because I like writing on my combat sheets and didn't want to mar my Monster Manual. Now I do it because of syntax highlighting. I use a text editor called vim. It lets me define coloration for blocks of text depending on what patterns appear in the text. Here's an example of a 4e monster I made up: Belligerent Sysadmin PNG
I also made a script that reads in my combat sheets and outputs a PDF with similar syntax highlighting, but with a white background for easy printing: Belligerent Sysadmin PDF
Unfortunately neither of those links shows an interrupt power, but I think you get the point. Interrupts show up in light blue. When something happens to the NPC I scan my combat sheet and read all the triggers on light blue powers. If they match what just happened, we have an interrupt.
I don't expect my vim and pdf workflow to work for everyone (although I'd be happy to share my scripts if anyone wants them). But it works for me. I'm sure you can do something similar in your text editor of choice, although it may require coloring powers by hand.
Beyond that, fixing missed interrupts is one of the few things I'm willing to fudge in 4e. If I forget about an interrupt power I change it's trigger. I creature that gets a free basic attack when it's bloodied instead gets a free basic attack once per encounter when it's damaged.
-- Now, with scripts! --
4E2latex.pl (renamed -txt to .pl)
Vim syntax highlighter