If I ignore my inbox for just a couple of hours, it runs wild with dozens of messages, most of which are really just replies to a handful of original threads. If I'm lucky, I can sometimes read the last e-mail in the thread, which includes all the relevant replies, and ignore the earlier messages. That sort of shortcut is even easier with Outlook Thread Compressor, a free program that deletes those earlier, redundant messages from your inbox for you.
Outlook Thread Compressor really has two key benefits. Not only does it reduce the complexity of your inbox, reducing a dozen e-mails down to one long thread you read in a single entry, but it keeps the size of your inbox under control as well by deleting unnecessary e-mail. The program is smart; it won't delete messages that include attachments, and it won't eliminate messages that fork off into new directions.
You might be concerned that Thread Compressor actually deletes email from your inbox. I've been using it for a while and never found it to do anything worrisome, but there are quite a few safeguards anyway. It must be run manually, for instance -- it's an Outlook add-on, and you'll find it in the Tools menu after installing it. I run it daily, or whenever I've been away from e-mail for a few hours and return to an unruly inbox. You can choose which folders to run Thread Compressor on, and you can even specify options like ignoring e-mail from specific recipients and "out of office" messages. You can even use the program in training wheel mode -- namely, just marking messages as read rather than deleting them.
Installing Outlook Thread Compressor reveals its roots as an internal Microsoft tool that has never been officially released to the public. To get it working, you'll need to manually replace a handful of system files via the command line. The directions are straight-forward and I did it in about five minutes, but it's a little intimidating nonetheless. If this doesn't sound like your cup of tea, but you still want a way to reduce the size of your inbox, check out how to improve Outlook's performance by removing attachments.