Any tips for a Spike from buffy costume?

Like I said in your other question, work on your British accent. And be careful with bleach, unless you want it to look that way for a long time! The way I understand it, bleach is pretty much permanent, and you'd have to wait for your hair to grow out before it's gone. If you don't want permanent blonde hair, I'd go with one of the temporary (one-night) Halloween hair sprays. That's your safest bet. If your hair is naturally dark, you won't be able to dye it blonde temporarily; you can only dye hair a darker shade than it is. Good luck, and great costume idea!