Actually, I'm just proceeding extremely cautiously, and have been right from the start, since it was my goal from the beginning to try to get good at both.  The "key" is to not overload your mind by practicing so hard on new layout that you begin mixing the keys up in the other layout (the one that you already know).

When I learned Dvorak, for example, I went all-out... I tried to learn too much, too fast, and by the time I got through the mega-confusion from all the changed keys and could finally type halfway decently in Dvorak, my QWERTY skills were already rusty.  No doubt that of what's left of my QWERTY memory, some of it is being put to use as stepping stones to learning Colemak.

Also, the fact that I just learned and switched to another layout as of only about four months ago means that the experience is fresh in my mind so I knew what to expect this time.  I would therefore be able to use the knowledge gained from this experience to prepare for and plan my method of learning a secondary layout in such a way that is least-damaging to the layout I already know.  Or at least, that was my theory.  It seems to be holding true for the most part though... so far.

Yep, step by step, it's all coming ;)

You trooper, you. ;)

I think at this time, you might want to go back to a tutor, and do the agonisingly boring drills of say one row at a time, to reinforce those key mappings.

You can try this: write down some notes that remind you how to type better – purely subjectively, focusing on accuracy. Sounds very silly but this is what worked for me when I was starting. I would start typing in Typeracer and after one good race I would just start making mistakes – possibly out of impatience, as you say. So I wrote a note on my computer and looked at it when I started typing sloppily. You don't actually need to type it down, you can just make a mental note, of course. So here's what I had in my OneNote notebook:

– Keep accuracy above 98% (that's just for motivation at the end, try to see the number 98, even if I typed slower)
– Type fluently, don't do sudden bursts on common words that I know how to type with one roll.
– Keep my hands on place and try to stretch my fingers rather than moving the whole palms around – this sounds awful but I noticed it helped with my accuracy. It's just a matter of starting to do it, keep my accuracy high for a while, then relax and go to my normal way of typing while keeping error-free
– force myself to use both backspaces rationally depending on the next letter. That's usually handled well by the brain by itself but I found it good to force myself sometimes to consciously think which backspace is better. The right one can be pressed quicker in succession but the left one could be more beneficial in other situations. Also pressing both twice deletes 4 characters for the time it would take to delete only 2 if I used just one of them. Anyway, too much details...

These, as you can see, are so subjective and most probably wouldn't work the same way for you or anyone else, but you see the idea. Figure out some "rules" that help you guide yourself on how to type better – be it accuracy (probably is), rhythm, hand position, etc. Then look at them and try to follow them until you're happy with the result and slowly start raising the speed.

I remember that when things were very bad with accuracy I would intentionally force myself to type slower for a while while my hands start typing without the constant mistake interruptions, then slowly raising the speed.

I've been aiming for 98% as well.  And I've seen others suggest similarly.  But why this number in particular?  Ideally we should aim for 100, but when I tried http://play.typeracer.com/?universe=accuracy, it was infuriating.  Does 99%+ sacrifice too much speed?

I remember Gtypist being such a hard task master, in terms of accuracy, it was grueling, but hopefully it paid off for me in the long run.

When I was a couple of days into learning a new layout and touch typing.  It felt so slow and difficult.  At about 3am one morning I literally just tried to let go (and use the force,) and just threw myself at the keyboard with reckless abandon.  And in someway it helped momentarily, I cruised all the lessons in Ktouch.  Though mistakes were a plenty.  I think the next day I regressed, but I was pretty pleased with myself at the time.  I probably over did it.

Klavaro touch typing software, breaks practice into distinct areas: basics (teaching you the layout, row by row), adaptability (random chars/keys), speed (random words) and fluidity.  The speed exercise is less concerned about accuracy (95% with no error correction), whereas the fluidity exercises want a good rhythm and accuracy of at least 97%.  I like the mix, and each feel very different.

Gtypist insisted that I was accurate before moving onto the next exercise, and that could be agonizing.  In the end you were forced to take it slow and steady, else boredom would completly and utterly set in.  I can't remember that being fun.

Perhaps a mild electric shock upon an error would increase accuracy (kids don't try that at home)?   My system beep is pretty ugly - and feels like a jolt in itself.  Or possibly practice next to a sleeping partner, and give yourself good reason not to make a mistake - for fear of the wrath of waking them.

Forcing you to correct your errors, does encourage you slightly to do better, as otherwise it's a burden as you end up having to type more.  It's just particulary grueling when starting out.  The Klavaro speed exercise let's you type through your mistakes.   Though it can feel a little wrong!

Perhaps with regard to having two backspaces, just force yourself to use a different hand at a time for a while,  I've done that with the space bar,  once you are adept with it,  you can fall back to using both hands, you might find then that you'll naturally use the one that feels better.

I can't seem to get myself into the habit of using CTRL+Backspace, to delete a word backwards, in someways I'd be tempted to change that shortcut to not require a simultaneous key press.  It's quicker to type sometimes than to backspace a character at a time - then retype.  Navigation becomes the slowest part.  Being able to navigate faster (perhaps with a  mod), could really help there.   I'm not sure whether these further tweaks warrant learning as early as possible, or whether they just complicate matters.

Surely the dulcet tones of the model m can only help in giving (some)one a good night's sleep?

I hate to bump this back up when I don't know how much more practice I'll be able to put into Colemak in the coming weeks, but here's an update.

I have been doing 30-second and 1:45 typing tests the last several days at hi-games.net.  My speed seems to be 35-40 WPM briefly in the 1:45 test, typically tapering down to 30 or so, only occasionally a couple WPM lower.  My capability to correct mistakes is much faster than the last time I posted, and I seem to be able to do slightly longer runs without thinking about where the key is.  It seems that the confusion over keys has mostly went away since my break, and my accuracy has actually gone up from what I remember as well.  My 30-second tests tend to be in the mid-30s WPM range.

I have gone through a long period when I had to pause practice due to life getting in the way (about four months based on this topic's original post date), but once I resumed it seemed that my previous skills were intact and my speed was quickly resumed.  I might be reaching another of those times already, unfortunately... not sure yet. :/

In fact, my 1:45 test record is actually Colemak (although my profile is set to Dvorak), as I decided to use that test for specifically for Colemak practice for the time being:
http://hi-games.net/typing-test,105/watch?u=6657

Before I started practicing again, my record for the above test was 25 or 26 WPM.  So... a 4-5 WPM gain... IMO not bad, especially for a longer test.