Naramri,
I've been shooting LF for more than 30 years and still make mistakes like yours occasionally (luckily, not that often, and usually only after not shooting for a longer period - kind of getting used to things again mistakes).
At any rate, I have a mental routine I go through that is somewhat like shutterfinger's. Mine (when using a roll-film back) is:
After composing and focusing, I meter the scene and determine f-stop and aperture. Then I walk around to the front of the camera and do the following while looking back at the lens to make sure everything is functioning properly:
1. close the preview
2. set aperture and shutter speed
3. cock the shutter and fire it (this "warms up the shutter, ensures that the preview is closed and that I've set the right speed)
4. cock the shutter again.
Then, I walk to the back of the camera and:
1. mount the roll film back
2. pull the darkslide
3. make the exposure
4. re-insert the darkslide
5. advance the film*
(5a. repeat exposures if you shoot more than one frame per scene by simply exposing and advancing till you're done)
6. (most important) write down your data in your exposure record: subject, exposure data, filters, and especially, the frame(s) you've exposed!
*Shutterfinger likes to advance his film right before the shot, which is a good idea if film flatness is an issue. For me, however, it is more important that the frame I've just shot gets advanced and out of harm's way so I can't ruin it by accidentally pulling a darkslide, etc. It can sometimes be confusing knowing whether you've advanced or not... when in doubt, always burn a frame and advance. A blank frame is better than a double-exposure.
And, if you are keeping an exposure record (which is your best friend and will help you make improvements faster than anything else), you can always check it to see if you've advanced the film or not
If you do switch to sheet film, then using filmholders comes with its own set of potential errors. Greatest among them is forgetting to flip the darkslide when re-inserting to show that you've exposed that particular sheet, and then later accidentally double-exposing the sheet, thereby ruining two shots. I don't do this much anymore, but it took me a while...
Oh yeah, don't toss your trashed roll of film. You can use it to practice loading the development reel. There's a learning curve there too.
Hope this helps,
Doremus