IIRC, the burden of proof is on Subaru to demonstrate that the DIY maintenance (or lack thereof) caused the warranty failure, not the consumer's burden to prove they adequately maintained the vehicle.Yeah, keep a contemporaneous log of the work that you do, along with copies of receipts, and you'll be fine. I keep a written notebook in my car for that purpose, but you can just as easily do it electronically. Beyond any potential warranty issue, I think it's pretty useful and interesting for my own use ... and having it can also help if you sell the car privately, to show buyers that it's been maintained well.
Keep in mind, too, that a warranty claim can't be denied due to owner negligence unless the negligence is a proximate cause of the issue. If you forget to change the oil, for example, Subaru couldn't deny a claim for a failed CV joint, since the CV failure isn't tied to the lack of an oil change.