I will answer speedball_racer's questions first, and then post some of my own thoughts on the subject.
In terms of the durability of the inner elbow spot, I've noticed a similar phenomenon, speedball_racer. In fact, the outer half of the inside of my right elbow was my very first spot, and it served me well for probably around a month. For the first few days/maybe even a week or two, I couldn't notice any marks -- there was some general redness, but that was all.
However, it does take its toll underneath the skin. Eventually, the scar tissue under the skin built up. So while it looks okay on the outside, when you put the needle in you can feel the resistance of the scar tissue, and if there's enough of that, it can squeeze off the vein.
Such is how I ruined both of these sweet spots on the outer portions of the insides of both of my elbows. It was a very sad day. Now I've moved on to the inner portions of the insides of my elbows, and I can see what you mean by soreness, and apparently this area bruises easily for me too because I've been getting some gross colored bruises despite not (to my knowledge) missing or poking through a vein.
I also use 29 guage needles -- you using a fresh one each time (I live abount 90 minutes from the closest needle exchange, which is one of two in my entire state; it is illegal to purchase needles here and in all of my neighboring states except two, both of which are at least two hours away) will help immensely too, I imagine.
I have been using the same 10 for about a month now. Sigh. Need more.
Anyway, for skin health around the sites, I use the following, depending on some criteria:
1. Right after a shot if you missed, poked a vein, made an abscess, or anything of this nature: Heat (heating pad, long shower, something) on and off the area fo 30-60 minutes. This is a pretty neat phenomenon, since (as another poster mentioned), if you've got any drug left over in the site where you missed, it will be rapidly absorbed into your system with the application of heat. This phenomenon occurs because heat expands the cappilaries leading to the site, allowing traffic in and out of the site for healing, etc. Which is why it also helps for bruising (bruises are just stagnant blood; this flows them away).
2. Once you are sure the puncture has healed, moisturizing lotion with antioxidants and skincare substances. I use a lotion of Vitamin E with Aloe Vera, however some people swear by these ridiculous home brewed concentrations of 20 different antioxidants (royal jelly, eye of newt, etc.). I've never used one of these concoctions, but some people swear by them, so who knows. It's important to saturate your body and the site with antioxidants, since putting any foreign substance into your body (especially IV) will cause chemical reactions, which cause free radicals, which can cause cell death, cancer, and other bad things. Antioxidants bind with free radicals, neutralizing their dangers. So whatever you rub on the area, make sure it's got some topical antioxidants in it.
3. Don't use the same area over and over again -- find 3-4 areas and switch off, giving the others time to heal. This sounds like good advice but sometimes isn't practical -- you're shaking, sweating, and trying not to shit and site A isn't working for you, you're sure as hell going to go back to old faithful. But this will only work so many times before old faithful stops working, so be prepared.
4. Take a multivitamin. Drug use is a lot of chemical reactions, and there are a lot of free radicals in your system as a result. In addition, the body uses certain substances to deal with the drugs and therefore depletes those substances moreso than if one wasn't doing drugs. So it's always a good idea to ensure decent nutrition with a multivitamin, unless you KNOW that one of the contents inhibits the proper absorption of your DoC (e.g. some people hate vitamin C + heroin).
Hope these help someone out there.