Since the weight of your wig is mostly concentrated in the back, it's not surprising the wig kept slipping. Focus the hairpins in the areas where the most "stress" (weight) is located: directly around the ponytail and along the front edge (above the ears; if your wig doesn't have a skin top, a couple inches back along the top of your scalp). The trick is to be strategic with your placement; think of it like drops of glue or nails. If they're in the right places, you can get away with using fewer pins.
Hair pins can be a little difficult to find sometimes. If they don't have them in the hair care aisle at the supermarket, check your closest beauty supply store. I get most of mine at Sally's, along with a lot of my wig care stuff. They usually come in 2 colors, bronze and black (you might be able to find them in steel/silver if you're lucky). Bronze works for brown wigs and anything lighter. Black's okay but tends to show more unless you've got a really dark color wig. I generally buy bronze because they have the most versatility. Get a couple of packs. Like safety pins, you can never have enough.
Don't be afraid to twist and bend the pins to fit your needs. They're really cheap.
nikkiolie: What are you using to style the wig? For spikes I use Aquanet hairspray, the strongest hold (and cheap, $2 at the grocery store). A lot of people also use got2b Glued Freezing Spray, which I've tried. It's also quite strong, though more expensive. Pretty much all you need is a super-hold hairspray. Style your spikes and blast the crap out of them.
Gel's not very good with wigs, though a lot of people try it. It has a tendency to melt off the wig fiber in the heat and leave you with limp bangs/spikes.
I use pomade when I want a softer style (separating or chunking), and then a light mist of hairspray to hold it in place.
