Personally, I have an account with
www.vankampen.com - they allow me to automatically withdraw a specific amount from my account monthly, weekly, or however I want to set it up.
Another option you may consider if you are looking to invest your money in specific increments would be to open a savings account (I'd recommend ING) that has a decent interest rate as a place to put your money making it more difficult for you to touch. Then, every six months or so you'll have around $3000.00 to put in an investment of your choice.