I think left foot 'cross training' is one of the best ways to improve your left foot technique.
I mean that in a few ways. Here's one of the best examples off of Steve Smith 'History of the US Beat' DVD.
http://www.youtube.com/watch?v=Wqj3p8rF4ew
Steve has you transitioning between (3) of the most established techniques of playing bass drum. I've been playing this exercise for awhile and have seen some improvement - although my double bass pedal playing is almost non existent on my gigs. 100% non existent on gigs where I'm being paid.
Here's another type of approach from Derrick Pope albeit with a little less finesse than our esteemed and more experienced statesman Steve Smith:
http://www.youtube.com/watch?v=SSKS9a9NuPc
Most of the posts I see about trying to improve left foot/double bass speed, many being from beginning to intermediate players, have a few things in common. The first being that they aren't addressing the concept of more hours and time = better results. I'm not a heavy metal player or fan but I've caught some parts of interviews with Chris Adler and Jason Bittner. Both allude heavily to the fact that you have to put in an incredible amount of time on improving speed - while playing consistently, evenly and with at least acknowledging if not concentrating on technique. I think the aforementioned posters in this paragraph just don't take that seriously enough and think that somehow 'that's just crazy' or 'that may be good for them' etc. Ask yourself the question: How bad do I want to get faster? What's an unreasonable amount of time to dedicate to specifically practicing toward the goals I have? If it takes much longer than I thought, how am I going to stay encouraged and find MORE time (blow off all TV, surfing web, texting, girl/boy friends, parties, socializing, etc) to put into it. Ex. Chris Adler seems to say that he would practice for DB speed at a certain bpm - for an hour. Then move the metronome up 1-2 clicks and then - practice for an hour. Can you do this without suffering from boredom? When you hit the wall for where you are at, can you hang with the promise that it will come eventually over a long amount of time?
This is the kind of the no-pay work ethic we have to embrace when going for these goals. I know at my age and with my family responsibilities I won't be able to achieve all of the goals I'd like to (unless I marry into money). Prioritization is key for me. Quality for my present gigs comes first then other stuff. If a player has a lot of free time or is willing to make the time, they can advance faster than me within the same timeline (ex. over 2 weeks to 1 month)
Re:Numberless' post
IMHO: As much as thinking creatively and outside the box (not using HH) is a good way to achieve new ideas, not employing facility on an important part of the drum set is a good way to paint yourself into a corner later on. Of course wanting to achieve facility in a rock situation with hi hat and DB (Chris Adler) or in an extreme drumming (Donati, Lang) situation as opposed to a jazz situation with single bass drum and hi hat ( Jack DeJohnette, Tony Williams, countless others) should have a different practice emphasis, However, blowing off decent or good facility on LF hi hat work is not the best approach for anyone. You also lose the 'cross training' benefit. The physical mechanics of hi hat and bass drum are different but the mental and coordination efforts will cross pollinate the other.
Keep going for the gold with dedication and time but don't expect miracles. Do expect and have faith that hard work over time will reap rewards.
Jim