I found that things "stick" in my brain and "pop up" at inconvenient times so that I don't forget. If you develop a system to get these tasks out of your head, and you use the system consistently and without fail, you might find that you will begin to trust your external system allowi your mind to become quiet. Has worked well for me.
The system I use is GTD-based, a little bit of Zen-to-done. Focus is learned; practise and you will get better. For digital collection I use tools such as:
GTD/ZTD for methodology and ideological theory, the "system" if you will.
OmniFocus and iCal for tracking projects and tasks, and scheduling deadlines (omnifocus can sync with iCal but not yet iCloud).
OmniFocus for iPhone and Calvetica for an improved iPhone cal app.
Flagging, smart folders and consistent archiving in Mail for my email - Omni can also use an email as an attachment to a todo item, so you can attach an email to help you know the context around a task, and therefore how to best accomplish it.
You can then archive the email so it's not in your inbox, in order to reduce clutter and use the inbox only for collecting items that still need your decision about what to do with them (do, defer, delegate or delete/archive).
I also use a tool called MailActOn which allows you to use keyboard shortcuts to quickly apply rules to email - a must have for efficient and quick culling of an overflowing email inbox. I use it a LOT.
Use your system. Learn to trust it. You won't trust it of you don't use it all the time. The iPhone with Omni helps you enter data while away from the laptop. Syncing allows your phone and laptop to stay consistent with tasks, emails and calendar items.
Finally, for digital documents I use a combination of Leap and Yep as well as some folder hierarchy to tag, find an organize digital content. I scan many of my documents and tag them for later searching. Helps me reduce my office down to nearly-paperless.
The most valuable tool of all: CrashPlan with unlimited backups. $50/yr to keep my computer backed up- including revisions of files. iCloud does the same for my phone (also with occasional backups to my computer) for free.
These are Mac tools btw.