I had a nice long-winded post that my computer ate. Here's the cliff-notes version:
* Make it harder to engage in the habit. Wearing gloves or socks in situations you'd normally consider picking. Choose gloves and socks you like and want to wear, so it's not a punishment. If you find yourself tempted to remove them, then you're probably going in for a good picking - the additional barrier will help dissuade you from doing it.
* Get help. Have the people you are around when you do it - if it's not just a private thing - give you a gentle reminder to stop when they see you doing it. You're probably doing it half absentmindedly, and the reminder alone might be enough to stop many instances of it.
* Figure out the places/activities where you engage in the unwanted behavior. Where possible, avoid those places and activities. Where not possible, adjust how you do them so it's harder to pick.
* Know that there's often a medical component to a lot of these problems - or there was when it started. Bring it up with your doctor.
* Make sure you're sleeping/eating/exercising appropriately. Also, vitamins! Your body will be unable to build and rebuild your skin without the proper elements, and this can often cause very minor irritations that encourage your picking.
* As PatrThom suggested, change it out. The fact that you've been able to give up significant self harm for this suggests you can probably do it again. Consider, for instance, carrying lotion in a small bottle with you everywhere. When you get the urge, or catch yourself picking, give yourself a little foot/hand rub with the lotion.
* Consider the 12 step program. The reason I bring it up is because most of this advice represents one or more tools these types of programs use. Even if you don't want to do the whole thing, and even if you don't consider this an addiction, the reality is these addiction recovery programs are largely self analysis and behavior modification systems. Dive deeply into the steps and you'll find worksheets, tools, thoughts, and ideas on how to figure out why you're doing what you're doing, and how to modify things - environment, self, etc - so that you can live life free from things you believe are holding you back. There are addiction recovery programs held throughout the US and world, and undoubtedly there's one near you (but also far enough that you wouldn't know anyone at the meeting if you wanted). For instance, the
LDS church's addiction recovery program is free, and probably has many groups near you. While you will get the obvious addictions - alcohol, drugs, pornography - you'll also find people trying to alter their eating habits, parenting skills, manage their anger, and breaking seemingly small habits such as yours. I'm not suggesting you need such a program, but the tools are free, and they can provide some significant help in changing your behavior, particularly for those things you can't see a way to change about yourself right now. It might be hitting a mosquito with a sledgehammer, but if this really bothers you then going over the booklet might give you a few avenues to pursue in change, and if you exhaust your other options, or just want to hit it as hard as you can, this is a pretty reasonable path. You can fully participate without disclosing the nature of the behavior you want to modify if that's important to you, and even going through the program without a specific goal will give you tools for self-analysis and behavior modification that you'll find useful the rest of your life, so it's really worth serious consideration.
* Take it to the next level and give yourself full manicures and pedicures once a month. Sand your skin with those pedicure rocks, trim your cuticles, paint your nails, the whole works. Not only should this scratch the itch to pick and decrease skin irritation, it will also look nice and when you get the urge you'll 1) be able to put it off until the next session and 2) know that picking would ruin the effort you put into the work in the first place.
* Consult with a beautician. They will reassure you that you aren't the only one, and will undoubtedly have tips that will help you understand how to treat your feet and hands so, at least, your body isn't trying to get you to engage in picking.
* Choose a cosplay that will require flawless hands and feet. Treat this as part of your preparation for that role - everything else you do for your cosplay is difficult and you figure out how to get it done, if you apply those same desires and goals to this you might have enough motivation to keep yourself in check long enough to break the habit.
No, really, this is the cliff note version.